Not needed after ldap_int_thread_userctx_t.ltu_id was added and
_pool_wrapper took care not to update thread_keys[] during pauses.
Fix bogus thread_keys[].ctx comment.

cleanup correctly

- Move problematic mutex_unlock()-mutex_lock() into the unused #ifdef
  LDAP_PVT_THREAD_POOL_SEM_LOAD_CONTROL and add a FIXME comment inside.
- Delay ltp_starting--; until pool_wrapper().  Nonzero value could otherwise
  no longer be exposed when !defined LDAP_PVT_THREAD_POOL_SEM_LOAD_CONTROL.

causes ldap_pvt_thread_self to be called with the wrong prototype.

That can cause OpenSSL to use a garbage value, e.g. if the unsigned
long it expects takes two words but ldap_pvt_thread_t is an int.

I'm fixing it in HEAD now and also provoking an error if unsigned
long cannot hold a ldap_pvt_thread_t.  Otherwise it can silently
compile to broken code.  Maybe the latter should go in configure,
but since OpenSSL presumably breaks anyway if that fails I don't
see much point at this time.

Remove a bogus 'waiting' message; the script does not sleep there.

re-parse ID from passmod request, if necessary (ultimately fixes ITS#4964, along with fix to ITS#4973)

LBER_ALIGNED_BUFFER(OPERATION_BUFFER_SIZE), as discussed in ITS#4078.
Add a corresponding SyncOperationBuffer in overlays/syncprov.c.

Also plug a memory leak, set state vars even when unused for the sake of
debugging, and tweak some readability issues & data types.