recovery, tolerate clustered index records whose externally stored
columns have not been written.  This should remove the assertion failures
that were reported as Mantis issue#58, issue#62, issue#64.

trx_is_recv(): New function: TRUE if this transaction is rolling back
an incomplete transaction in crash recovery.

enum trx_rbmode: Rollback modes: no rollback, normal rollback, crash recovery.

btr_cur_pessimistic_delete(), btr_free_externally_stored_field(),
btr_rec_free_externally_stored_fields():
Replace the ibool parameter with enum trx_rbmode.

btr_free_externally_stored_field(): If field_ref is zero, return
but assert ut_a(rbmode == RB_RECOVERY).  Unless InnoDB has crashed
while inserting a clustered index record, field_ref should not be zero.

btr_rec_free_updated_extern_fields(): Add the parameter enum trx_rbmode.

btr_cur_pessimistic_update(): Pass the rbmode parameter to
btr_rec_free_updated_extern_fields().

row_undo_ins(), row_undo_mod_upd_del_sec(): If row_build_index_entry()
fails, assert trx_is_recv() and skip this secondary index.

row_undo_mod_upd_del_sec(): Empty the heap at the end of each loop
iteration in order to conserve memory and to reduce the number of
low-level memory allocations.