limbo: speed up synchronous transaction queue processing

This patch optimizes the process of collecting ACKs from replicas for
synchronous transactions. Before this patch, collecting confirmations
was slow in some cases. There was a possible situation where it was
necessary to go through the entire limbo again every time the next ACK
was received from the replica. This was especially noticeable in the
case of a large number of parallel synchronous requests.
For example, in the 1mops_write bench with parameters --fibers=6000
--ops=1000000 --transaction=1, performance increases by 13-18 times on
small clusters of 2-4 nodes and 2 times on large clusters of 31 nodes.

Closes #9917

NO_DOC=performance improvement
NO_TEST=performance improvement

(cherry picked from commit 4a866f64d64c610a3c8441835fee3d8dda5eca71)