snapshot: review fixes

- add forgotten latch.h, fix latch.h header guards
- make sure the stuff in thread is thread-safe and exception-safe
- remove snapshot_pid and update the docs.
- remove empty lines/trailing spaces
- move xlog_open()/xlog_close() to the snapshot thread
- use cord_costart() to have fiber infrastructure
 in the checkpoint thread, rather than run checkpoint
 in sched fiber
- remove #include <fiber.h> from memtx_engine.h, it's not
  needed there
- remove errNo() method from base class Exception, anything
  with an error no inherits from SystemError. Moreover,
  now that we have a rich exception infrastructure at our
  disposal, we should switch to using it in engine API,
  so in future returning error no will be replaced
  with exceptions.