replication: fix race in accessing vclock by applier and tx threads

When applier ack writer was moved to applier thread, it was overlooked
that it would start sharing replicaset.vclock between two threads.

This could lead to the following replication errors on master:

 relay//102/main:reader V> Got a corrupted row:
 relay//102/main:reader V> 00000000: 81 00 00 81 26 81 01 09 02 01

Such a row has an incorrectly-encoded vclock: `81 01 09 02 01`.
When writer fiber encoded the vclock length (`81`), there was only one
vclock component: {1: 9}, but at the moment of iterating over the
components, another WAL write was reported to TX thread, which bumped
the second vclock component {1: 9, 2: 1}.

Let's fix the race by delivering a copy of current replicaset vclock to
the applier thread.

Also add a perf test to the perf/ directory.

Closes #7089
Part-of tarantool/tarantool-qa#166

NO_DOC=internal fix
NO_TEST=hard to test

(cherry picked from commit ddec704e)