ITS#7961,#7987 Re-fix txn init.

More fallout from 4d02c741b120786df1b87ee9ed49c1d3f9bc7522:
Don't modify another thread's write txn. Reinit me_txn0 fully.
***  Merge remote-tracking branch 'origin/mdb.master' into 2.4-merge.mdb
[Suggestion] Simplify recent changes a bit.

Or maybe it's more readable if struct MDB_pgstate
goes away, now that it only has two members.
[Review comment] Fix robust mutexes - repair mti_txnid.

Broken by e3b6c359a935283c21e1fd7d03b790d87d53b933, if
commit() crashes in write_meta before setting mti_txnid.

[Clarify whatever is unclear about the /*comment*/, since
you removed that code?  Unless that was just an accident.
See <http://folk.uio.no/hbf/unrobust.c>.]
mdb_strerror: Silence warning @ Windows
[Review] mdb_page_alloc(): Use last page (off-by-one error)

[...or is an unused page at the end a feature, to protect LMDB and
next mapped page from broken pointers that walked past the boundary?]
Fix comments
[Partial] Clean up SysV semaphore code.

Change MDB_LOCK_FORMAT to differ from Posix semaphores/mutexes.
Do not use array/member length 0 for MDB_txninfo.mt2.

[However, this feature is broken.  It can break the datafile
the same was as robust mutexes without cleanup, see
<http://folk.uio.no/hbf/unrobust.c>.
Possible fix: Let MDB_USE_SYSV_SEM omit mti_txnid and always
use pick_meta().  Provided the unportable __sync_synchronize()
in write_meta() is cleaned up and not ifdefed out (ITS#7969).]
Fix pointer incompatibility issue on some platforms

Don't assume tv_sec is a time_t.
closedir() replacement function should return a result

return success, ignoring result of FindClose
ITS#7965 mdb_dn2id: heap corruption due to returning a reference to the local variable.
ITS#7967 fix abandon regression

From ITS#7712, avoid double-free of request
Fix a1e3b1cf3c5991ef550f608ee816e9274e547dda

copy/paste error
Fix typo in cancel config
ITS#7968 syncprov_matchops(), filter.
lmdb: more usefull info from mdb_stat tool.
lmdb: ITS#7974 get a lag of the reading for dreamcatcher.
slapd: ITS#7974 dreamcatcher feature.
lmdb: ITS#7974 oomkiller feature.
slapd: ITS#7974 oomkiller feature.