Add partial 'tcbdbtune' support

This patch was taken from Incubaid/arakoon@0c548cfe971efb7 and applied
manually.

See: https://github.com/Incubaid/arakoon/commit/0c548cfe971efb78d3700e7252b09ce8d1eb7086
Merge pull request #3 from NicolasT/ARAKOON-373

Add partial 'tcbdbtune' support
now depends on Incubaid's tokyocabinet fork.
opt needs to be in interface ...
using 'camltc-0.6.0'-tagged tokyocabinet
0.6.0 in META
exposed _tranbegin, _trancommit and _tranabort
Merge pull request #4 from domsj/expose-transactions

exposed _tranbegin, _trancommit and _tranabort
Add binding for `tcbdbflags`