info: don't use wal_checkpoint() to display the current vclock

Using wal_checkpoint() incurs an extra and unnecessary overhead
in tx thread. Monitoring inquiries are supposed to be quick
and incur close to no ovrhead on tx thread.

The state of replicaset vclock is good enough for monitoring.

Revert back the test results broken by
f2bccc18 by @rtsisyk
The original results were correct, as indicated by the comment
not changed by f2bccc18