There's several important stages of recovery of database: loading
from snapshot, then loading from WAL(s) and then normal operation.

Introduce a global recovery state that shows this stage.

Note that there's already a recovery state in memtx engine which is
very close but still different to the new introduced state. That
state in memtx is a private property of memtx that internally shows
initialization status of memtx spaces and indexes. Memtx can set the
value for convenience' sake, for example it can jump directly to
MEMTX_OK before snapshot loading in case of force recovert.

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