recovery: fix recovering from 1.6 xlogs/snaps

Tarantool 1.6 didn't rotate xlogs on snap creation.
This means, that Tarantool 1.6 data dir may contain files like
0...0.xlog and 0...01.snap, where 0...0.xlog covers changes both before
and after the snap creation.

New Tarantool versions fail to recover from such files: they recover the
data correctly, but fail to update the vclock accordingly.

Let's allow recovering from such files and update the vclock
accordingly, but only when force_recovery is set.

Patch 634f59c7 ("recovery: panic in case of recovery and replicaset
vclock mismatch") added the panic, which helped to discover the issue.
Before that patch Tarantool would silently recover with an outdated
vclock.

Also, while we're at it, refactor the check leading to a panic: it never
happens after hot_standby, so move it into "else" branch of hot_standby.

Closes #6794

(cherry picked from commit 8c8b7739)