memtx: fix force recovery handling

Force recovery needs to follow the following logic: any unsuccessful system
space request must make recovery fail (including failure to decode an xrow,
when we are not sure we have finished processing system space request). If
the request is a non-insert one (e.g., raft or synchro) or addresses a user
space, it means we have finished processing system space requests, and from
this moment force recovery can be enabled — change the behaviour
accordingly.

We assume the request order in the snapshot is the following:
1. system space requests;
2. user space requests;
3. non-insert requests (e.g., raft or synchro).

Refactor the force recovery logic: add a enumeration to track snapshot
recovery state and add a new diagnostic for the case when the snapshot
contains has no system spaces.

Closes #7974

NO_DOC=bugfix