(#583) - compact callback after compaction's done

Now http adapter behaves like others. The code manually pings
database to check if it's done compaction.
(#583) - uncomment compaction test

This test was failing due to http compaction took some time
and callback was fired before it finished. Now it's fixed
(#583) - add compaction documentation
(#583) - ping remote server correctly

Better now:
1. starting pinging _after_ the compaction started
2. try again _only_ if we got answer
Merge pull request #617 from neojski/583-call-compact-callback-when-done

(#583) - call compact callback when done