Clarify mdb_dbi_open doc

The concurrency restriction is on threads within a single process.
Multiple processes can use mdb_dbi_open without affecting each other.
make clean: remove *.lo
Fix MDB_APPEND doc

We actually compare the new key against the last key, to prevent
corruption.
Merge remote-tracking branch 'origin/mdb.RE/0.9' into OPENLDAP_REL_ENG_2_4
ITS#8006 Use rootdn consistently on internal ops
ITS#8006
*** Peter-Service build script.

   On the origin/OPENLDAP_REL_ENG_2_4 at 2014-12-14.
Merge remote-tracking branch 'origin/mdb.master' at 2014-12-14.
master-minor: Use OpenSSL md5/sha1
master-minor: Updates for new servers
lmdb-extra: 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?]
slapd-extra: ITS#7968 syncprov_matchops(), filter.
slapd-extra: mdb-backend error logging.
lmdb: more usefull info from mdb_stat tool.
lmdb: ITS#7974 get a lag of the reading for dreamcatcher.
lmdb: lock meta-pages in writemap-mode to avoid unordered write.

Without locking the meta-pages may be writen by OS before data pages,
in this case database would be inconsistent.
back-mdb: checkpoint-interval in seconds instead of minutes.
lmdb: auto-sync with kbytes threshold (aka checkpoint by kbytes).
back-mdb: support for "checkpoint kbytes" config-option.
lmdb: properly sync meta-pages in mdb_sync_env().

Meta-pages may be updated during data-syncing in mdb_sync_env(),
in this case database would be inconsistent.

Check-and-retry if lead txn-id changed during flushing data in mdb_sync_env().