add non generic async versions of deleteallasync and droptableasync
add non generic async versions of deleteallasync and droptableasync

molinch/SQLite.Net-PCL,oysteinkrog/SQLite.Net-PCL,mattleibow/SQLite.Net-PCL,caseydedore/SQLite.Net-PCL,fernandovm/SQLite.Net-PCL,TiendaNube/SQLite.Net-PCL,kreuzhofer/SQLite.Net-PCL