Add unit test for FrontedMetadata memory leaks

This memory leak occurs when write and read-only transactions
intertwine, leading to sparse range set.
We need to be able to request frontend metadata from shards,
so we can see whether it converges to a single range.

Also rework testing datastores, so we can inject custom testing
implementations allowing us to retrieve frontend metadata.

JIRA: CONTROLLER-1879
Change-Id: Ia8f350be7831b0c8f6846ee6fa41a665d1e191f4
Signed-off-by: Tomas Cere <tomas.cere@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>