Release OCaml runtime lock around `tcbdbdefrag`
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.

toolslive/camltc,toolslive/camltc