OpenTSDB: Made api query non-blocking. (https://rbcommons.com/s/pepperdata/r/3858)
- Running a OpenTSDB query was blocking because of the type management issue of
annotation and query results as data points. - Introduced TypeCastingCB as a
convenient class to cast types of deferred objects. - Introduced a nested class
AsyncQuery to asynchronously process annotation and query results together.