applier: move applier_thread struct to header

The next commit will need to access it from applier_signal_ack, which is
defined upper. We could move applier_thread upper in the C file, but I
don't see any point to hide this struct: it brings no new dependencies.
Let's keep all applier structs together in the header file - it makes
the code easier to follow.

NO_DOC=refactoring
NO_CHANGELOG=refactoring