[REFACTOR] libsnapshot: Add InitiateMergeAndWait

Move snapshot_ctl's merge command to SnapshotManager::
InitiateMergeAndWait function so that tests can use
it too.

Test: libsnapshot_test
Bug: 143551390

Change-Id: I0936c262afaca7ba445ee18465dca4e16b9416ad
4 files changed