snapuserd: Refactor client to allow persistent connections.

This refactors SnapuserdClient so it retains a connection for its
lifetime. This allows SnapshotManager to ensure the daemon is running
and hold a connection open across all of its operations.

The main impetus of this change is to remove the ambiguity between first
and second-stage sockets. SnapshotManager should only ever connect to
the first-stage socket during first-stage init, or, to initiate the
"transition" step during second-stage init.

The transition steps are roughly:
 (1) Start second-stage daemon.
 (2) Load new device-mapper tables.
 (3) Connect second-stage daemon to new dm-user devices.
 (4) Activate the new tables, flushing IO to the first-stage daemon.
 (5) Send a signal to the first-stage daemon to exit.

This patch makes it easier to hold these two separate connections.

Bug: 168554689
Test: manual test
Change-Id: I51cb9adecffb19143ed685e0c33456177ec3d81f