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 master in 31 minutes and 31 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #2362
build-gnutls-mit

00:30:09

passed #2360
build-no-threads-no-slapd

00:03:28

passed #2361
build-openssl-heimdal

00:31:31