Add a GC to the lmdb backend

Here is a first PoC to add a gc command to the lmdb backend. The current strategy for the GC is "stop-and-copy", e.g:

• the GC cannot run concurrently with any other writes
• we create a temporary file to promote living objects
• once scanning is done and all living objects are promoted, we rename the database file and re-open the database handler

Also, it seems that the current implementation "freeze" the application (e.g. CTRL-C won't work anymore) so there is probably something to do with signal hander.

Few things needs to be cleaned up before merging, but I wanted to have an early feedback.