Rewrite checkpoint daemon in C

Long time ago, when the checkpoint daemon was added to Tarantool, it was
responsible not only for making periodic checkpoints, but also for
maintaining the configured number of checkpoints and removing old snap
and xlog files, so it was much easier to implement it in Lua than in C.
However, over time, all its responsibilities have been reimplemented in
C and moved to the server code so that now it just calls box.snapshot()
periodically. Let's rewrite this simple procedure in C as well - this
will allow us to easily add more complex logic there, e.g. triggering
checkpoint when WAL files exceed a configured threshold.

Note, this patch removes a few cases from xlog/checkpoint_daemon test
that tested the internal state of the checkpoint daemon, which isn't
available in Lua anymore. This is OK as those cases are covered by
unit/checkpoint_schedule test.