extract attachment from hive.contexts to hive.contexts_attachment

There was a problem with starving removing reversible data
because in some rare situations lock was taken to often and
prevented 'set_irreversible' function to clean up obsolete blocks.
To ensure that lock is held only when attachment was changed, column
is_attached was moved to separated table.

Problem was obserwed in system test 'replay_with_app'. In the test log
there was iformation about minimal reversible block, which was few
tousands behind the head block;