Improve MySQL sync speed

[gregharvey]

We should separate the unzipping action from the MySQL import because bzcat of a zip file piped to mysql is really slow!

[galooph]

@tymofiisobchenko has been working on this – see #337 and #338

[tymofiisobchenko]

#291 Splitting tasks into separate ones gave a small advantage (which is also difficult to measure, because the database involved in the comparison is constantly growing). At least the load has noticeably reduced, before these changes the server was almost unresponsive during the sync process. The main advantage was in the moving from bzip2 to gzip (archive size differs insignificantly, but the speed in times).