datastore: Keep a snapshot of the latest branch

datastore: Keep a snapshot of the latest branch

This avoids making immutable copies of the same branch over and over.

Also use the latest branch directly in the node insertion instead of
the getter.

This change alone removes all performance impact of the datastore
when building.

Change-Id: If85ae9bded364cfd6f1243416d025df6307cf909
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/91755
Reviewed-by: Hudson CI
Reviewed-by: Loic Prieur-Drevon <loic.prieur.drevon@ericsson.com>