recovery: make it yield when positioning in a WAL

We had various places in box.cc and relay.cc which counted processed
rows and yielded every now and then. These yields didn't cover cases,
when recovery has to position inside a long WAL file:

For example, when tarantool exits without leaving an empty WAL file
which will be used to recover instance vclock on restart. In this case
the instance freezes while processing the last available WAL in order
to recover the vclock.

Another issue is with replication. If a replica connects and needs data
from the end of a really long WAL, recovery will read up to the needed
position without yields, making relay disconnect by timeout.

In order to fix the issue, make recovery decide when a yield should
happen. Once recovery decides so, it calls a xstream callback,
schedule_yield. Currently schedule_yield is fired once recovery
processes (either skips or writes) WAL_ROWS_PER_YIELD rows.

schedule_yield either yields right away, in case of relay, or saves the
yield for later, in case of local recovery, because it might be in the
middle of a transaction.

Closes #5979