libsnapshot: Get DaemonTransition test passing again.

This fixes a number of small bugs in libsnapshot. It also refactors the
handler list a bit. Previously, it was a list of unique_ptrs. Now it is
a list of shared_ptrs to simplify ownership.

Additionally, Snapuserd is now keyed solely on the misc device name.
This allows two identical snapshots to run in the same daemon, with
different control names (a scenario that comes up in the
DaemonTransition test). As part of this change, the two-stage
initialization process has been refactored slightly. The "init" message
sets all the device paths, and the "start" message needs only the misc
name.

Both the init and start messages now validate that no duplicate handlers
exist, and that we're not overwriting any previous thread.

This cleanup also fixes a bug in DmUserHandler cleanup - if a control
device shut down raced with WaitForDelete(), the std::thread object
would delete without a call to detach() or join(). In the new
RemoveHandler(), we now correctly detach() in this scenario.

This also fixes a bug where, if a COW had no partition component (it
only resided on /data), the second-stage transition would fail because
it used the wrong device-mapper name.

Bug: N/A
Test: vts_libsnapshot_test
Change-Id: Ib4a281a3b5fe665c727c7077672e3c6b0b3abdba