misc fixes of execution of UNDOs in the UNDO phase:
- into the CLR_END, store the LSN of the _previous_ UNDO (we debated
what was best, so far we're going with "previous"; later we can change
to "current" if needed), and store the type of record which is being
undone (needed to know how to update state.records when we see the
CLR_END during the REDO phase).
- declaring all UNDOs and CLR_END as "compressed"
- when executing an UNDO in the UNDO phase, state.records is updated
as a hook when writing CLR_END (needed for "recovery of the state"),
and so is trn->undo_lsn (needed for when we have checkpoints).
- bugfix (execution of UNDO_ROW_DELETE didn't store the correct checksum
into the re-inserted row, maria_chk -r thus threw the row away).
- modifications of ma_test1: where to stop is now driven by --testflag;
--test-undo just tells how to stop (flush data, flush log, nothing).
- ma_test_recovery: testing of the UNDO phase, more testing of the
REDO phase, identification of a bug.


storage/maria/ma_blockrec.c:
  - bugfix: execution of UNDO_ROW_DELETE didn't store the correct
  checksum into the row (leading to "maria_chk -r" eliminating the
  re-inserted row, net effect was that rollback appeared to have
  rolled back no deletion). Reason was that write_block_record() used
  info->cur_row.checksum, while "row" can be != &info->cur_row
  (case of UNDO_ROW_DELETE). After fixing this, problems with
  _ma_update_block_record() appeared; indeed checksum was computed
  by  allocate_and_write_block_record() while _ma_update_block_record()
  directly calls write_block_record(). Solution is to compute checksum
  in write_block_record() instead.
  - when executing an UNDO, we now pass the LSN of the _previous_ UNDO
  to block_format functions. This LSN can be 0 (if the being-executed UNDO
  was the transaction's first UNDO), so "undo_lsn==0" cannot work
  anymore to indicate "this is not UNDO work". Using undo_lsn==LSN_ERROR
  instead (this is an impossible LSN).
  - store into CLR_END the type of log record which was undone
  (INSERT/UPDATE/DELETE); needed for Recovery to know if/how it has
  to update state.records if it sees this CLR_END in the REDO phase.
  - when writing the CLR_END in _ma_apply_undo_row_insert(),
  the place to store file's id is log_data+LSN_STORE_SIZE.
  - in _ma_apply_undo_row_insert(), the records-- is moved
  to a hook when writing the CLR_END (this way it is under log's mutex
  which is needed for "recovery of the state")
storage/maria/ma_loghandler.c:
  - all UNDOs, and CLR_END, start with the LSN of another UNDO; so
  we can declare them "compressed".
  - write_hook_for_clr_end() to set trn->undo_lsn (to the previous
  UNDO's LSN) under log's lock (like UNDOs set trn->undo_lsn under log's
  lock), and also update, if appropriate, state.records.
  - reset share->id to 0 when deassigning; not useful for now but
  sounds logical.
storage/maria/ma_recovery.c:
  - if no table is found for a REDO, it's not an error; for an UNDO, it is
  - in the REDO phase, when we see a CLR_END we must update trn->undo_lsn
  and sometimes state.records.
  - in the UNDO phase, when we execute an UNDO_ROW_INSERT:
    * update trn->undo_lsn only after executing the record
    * store the _previous_ undo_lsn into the CLR_END
  - at the end of the REDO phase, when we recreate TRN objects, they
  have already their long id in the log (either via a
  LOGREC_LONG_TRANSACTION_ID, or in a checkpoint record), don't write
  a new, useless LOGREC_LONG_TRANSACTION_ID for them.
storage/maria/ma_test1.c:
  * where to stop execution is now driven by --testflag and not --test-undo
  (ma_test2 already has --testflag for the same purpose). This allows
  us to do a clean stop (with commit) at any point.
  * --test-undo=# tells how to abort (flush all pages (which implies
  flushing log) or only log or nothing); all such "ways of crashing"
  are tested in ma_test_recovery
storage/maria/ma_test_recovery:
  * Testing execution of UNDOs, with and without BLOBs.
  * Testing idempotency of REDOs.
  * See @todo for a probable bug with BLOBs.
  * maria_chk -rq instead of -r, as with -q it nicely stops on any
  problem in the data file (like the checksum bug see comment of
  ma_blockrec.c).
  * Testing if log was written by UNDO phase (often expected),
  not written by REDO phase (always expected).
  * Less output on the screen, compares with expected output in the end.
  * some shell thingies like "set --" and $# are courtesy of
  Danny and Pekka.
storage/maria/maria_read_log.c:
  when only displaying the records, don't do an UNDO phase
storage/maria/ma_test_recovery.expected:
  This is the expected output of a great part of ma_test_recovery.
  ma_test_recovery compares its output to the expected output
  and tells if different.
  If we look at this file it mentions differences in checksum
  (normal, it's not recovered yet) and in records count
  (getting a correct records' count when recovery starts on an
  already existing table, like when testing rollback,
  is coded but not yet pushed).