BUG-9054: do not use BatchedModifications needlessly

BUG-9054: do not use BatchedModifications needlessly

Transaction identifier, which is a required parameter for
BatchedModifications is a resource tracked on the backend and is
assumed to be allocated contiguously. Using BatchedModifications
to transport only a list of modifications means we are allocating
transactions IDs which we then never use.

This patch reworks the logic so it tracks modifications in a list
and allocates BatchedModifications only when we are ready to actually
commit something.

Change-Id: I3f71511cfd68e96e80790e69d28d083f195e5e12
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>