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