Use a single write txn
Remember oldest reader txnid

Don't walk the readers table unless we really need to.
More for me_pgoldest

Leave it in main env struct, it doesn't need to be rolled back
with nested txns.
Change retry to num times 60.  Testing shows that on a known dataset, this has the same growth behavior as 2.4.39, while num times 20 resulted in significant growth.
Merge remote-tracking branch 'origin/mdb.master' into OPENLDAP_REL_ENG_2_4
ITS#7937 flush data to socket
ITS#7937
2.4.40 release
Release 0.9.14
Merge remote-tracking branch 'origin/mdb.master' into OPENLDAP_REL_ENG_2_4
Silence some clang warnings
ITS#7941 fix for repeated tags

Make sure ntags isn't incremented if we're skippnig the tag
ITS#7941
Plug leak in 4d02c741b120786df1b87ee9ed49c1d3f9bc7522

Free writer txn on env_close
ITS#7942 plug leak in controls
Merge remote-tracking branch 'origin/mdb.master' into OPENLDAP_REL_ENG_2_4
ITS#7942
ITS#7942 tweak previous commit

Only free when cleaning up operation controls, not response controls.
(This only makes a difference on search requests, which may have
multiple responses with the same set of controls.)
ITS#7943 reinit txn flags

When reusing the main write txn
Merge remote-tracking branch 'origin/mdb.master' into OPENLDAP_REL_ENG_2_4