Partially Revert "ITS#8486 use kbtree for sessionlog"

This mostly reverts commit 1915cb96.
Too many concurrency issues. Retains the improvement to
syncprov_sessionlog_cmp
3 jobs for its8486c in 32 minutes and 37 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #2359
build-gnutls-mit

00:28:58

passed #2357
build-no-threads-no-slapd

00:03:38

passed #2358
build-openssl-heimdal

00:29:42