Add support for point-in-time recovery

We need to facilitate point-in-time recovery:
- a way to find out, looking at wal, what time it was started at and what time it ended up
- a way to specify time or lsn to which recover at start of the server, in the command line:
  * if this option is given, we should perhaps start in read-only mode, since we can't accept
updates as long as there are partial wals. then, we should exit read-only mode only
after save snapshot command, which will ensure that we can restart later on if necessary
from this new version of history. This problem needs more thinking.
 * the alternative would be to automatically delete the wals which are not necessary,
and truncate the wal whish is partially read
* rename the wals which are not read into .snap.corrupted.

Add a wal player to the command line client/libtnt library, so that we can
process a wal and send it to the server as a bunch of commands.