Call `tcbdbsetmutex` on created database handles

In order to use a TokyoCabinet `TCBDB` database in a multi-threaded
environment, `tcbdbsetmutex` should be called before the database file
is opened.

This commit adds this call to `bdb_make`, used by `Hotc.create`.
Release OCaml runtime lock around `tcbdbdefrag`

In order to be able to run `tcbdbdefrag` in a separate OS thread and
continue running OCaml code at the same time, the runtime lock should
be released. Especially since `tcbdbdefrag` can take quite some time
to complete.

This commit adds the appropriate release & acquire calls.
Merge pull request #8 from NicolasT/master

Enhance thread-awareness