wal: do not promote wal vclock for failed writes

Wal used to promote vclock prior to write the row. This lead to a
situation when master's row would be skipped forever in case there is
an error trying to write it. However, some errors are transient, and we
might be able to successfully apply the same row later. So we do not
promote writer vclock in order to be able to restart replication from
failing point.

Obsoletes xlog/panic_on_lsn_gap.test.

Needed for #2283