snapshotctl don't auto-merge.

update_engine is now responsible for initiating the merge. snapshotctl
becomes a debugging tool for libsnapshot now.

Bug: 147696014
Test: libsnapshot_test

Change-Id: Ia2527a35e0c0f0789dbe5c477e174663ef406903
Merged-In: Ia2527a35e0c0f0789dbe5c477e174663ef406903