snapuserd: Remove ambiguous BufferSink in workers.

BufferSink is not needed/used in all worker types, so move it explicitly
to MergeWorker. This also moves the sizeof(dm_user_header) computation
to BufferSink::Initialize.

Bug: 288273605
Test: snapuserd_test
Change-Id: I8fcea6f0e587ca6d8672df5ec58c4d4d14d4b72e