BUG-5280: add frontend state lifecycle

When transitioning between roles we need to take care of proper
handling of state known about frontend. This patch adds
the leader/non-leader transitions, creating the state
from FrontendMetadata and forgetting it when transactions are
committed.

Our replicated log needs to grow more entries to accurately
replicate the state of the conversation between the frontend
and backend, so if a member becomes the leader it has
an understanding of which transactions and transaction
chains have been completed (aborted, committed, purged). These
are replicated before a response is sent to the frontend, so
if a leader before they replicate successfully, the frontend
will see them as a timeout and retry them (and be routed to the
new leader).

Both leader and followers are expected to keep the metadata
handy: the leader keeps for the purpose of being able to generate
a summarized snapshot. The followers keep it so their metadata
view is consistent with the contents of the data tree.

Change-Id: I72eea91ee84716cdd8a6a3521b42cca9a9393aff
Signed-off-by: Robert Varga <rovarga@cisco.com>