master, metalogger: Fix how `sessions' downloading is handled

Downloading sessions file can happen at any time (asynchronously)
which means it can also happen virtually at the same time
as potential error in applying changelog.

Up till now we assumed that at end of sessions download we are
in 'download state' which was not quite true if changelog apply
error occurred.

Additionally we shall prevent sessions file to be downloaded
at all before we safely enter `synchronized state'.

Change-Id: I55d76735f81d84d60689e64447fc99195e9a2d43