add continuation support to applyRevisionFile()

add continuation support to applyRevisionFile()

even if the plan is to migrate fully to Room, writing DB structure
revisions in a series of SQL files is much more convenient compared to a
sequence od db.execSQL() calls with very long arguments

except that writing SQL text that is very long is a problem for the SQL
files too. Continuations solve this in a way, much better than splitting
long SQL arguments to execSQL() in a series of concatenated strings