Concurrent execution of statements which require non-table-level
write locks on several instances of the same table (such as
SELECT ... FOR UPDATE which uses same InnoDB table twice or a DML
statement which invokes trigger which tries to update same InnoDB
table directly and through stored function) and statements which
required table-level locks on this table (e.g. LOCK TABLE ... WRITE,
ALTER TABLE, ...) might have resulted in a deadlock.

The problem occured when a thread tried to acquire write lock
(TL_WRITE_ALLOW_WRITE) on the table but had to wait since there was
a pending write lock (TL_WRITE, TL_WRITE_ALLOW_READ) on this table
and we failed to detect that this thread already had another instance
of write lock on it (so in fact we were trying to acquire recursive
lock) because there was also another thread holding write lock on the
table (also TL_WRITE_ALLOW_WRITE). When the latter thread released
its lock neither the first thread nor the thread trying to acquire
TL_WRITE/TL_WRITE_ALLOW_READ were woken up (as table was still write
locked by the first thread) so we ended up with a deadlock.

This patch solves this problem by ensuring that thread which
already has write lock on the table won't wait when it tries
to acquire second write lock on the same table.

mysql-test/r/lock_sync.result:
  Added test case for bug #45143 "All connections hang on concurrent
  ALTER TABLE".
mysql-test/t/lock_sync.test:
  Added test case for bug #45143 "All connections hang on concurrent
  ALTER TABLE".
mysys/thr_lock.c:
  Ensured that thread can acquire write lock on the table without
  waiting if it already has write lock on it even if there are other
  threads holding write locks on this table (this is normal situation
  for, e.g., TL_WRITE_ALLOW_WRITE type of lock).
  
  Adjusted comments to better explain why it is OK to do so and added
  asserts to prevent introduction of scenarios in which this can cause
  problems.