Don't change mc_dbx because we would need to undo the change if
the txn aborts. Make the fix (for get) match existing code for put.

This reverts commit 47c4e747.

Revert "ITS#8117 fix INTEGERDUP compare"

This reverts commit ba45e0c0.

Fix mdb_txn_begin(&read-only txn) calls. Test mdb_env_set_maxreaders().
Rename DBI open/close functions. Move mdb_dbi_close() out of txn.

(Probably fixes the ITS, definitely fixes a bug) when collapsing
the root page, fixups of other cursors was incomplete.

Copied from 58ddb552.

Leonid Yuriev authored Oct 17, 2014 and Hallvard Furuseth committed Jan 14, 2015

Inconsistent reading & SIGSEGV due to the race condition.

Leonid Yuriev authored Oct 17, 2014 and Hallvard Furuseth committed Jan 14, 2015

Globally shared fields of meta-data were not 'volatile'.
Code from 9a80a8a8,
except the unportable __synchronize().

Check for ext3/ext4 fs, then check kernel version.