DRILL-3566: Fix: PreparedStatement.executeQuery() got ClassCastException.
Main:
Restored DrillResultSetImpl(...)'s statement parameter from overly
restrictive DrillStatementImpl to AvaticaStatement and removed caller
cast that was throwing. (Relatedly, adjusted getStatement() and moved
internal casting from statement to connection.)
Added basic test of querying via PreparedStatement. [PreparedStatementTest]
Added some case test of statement-creation methods. [ConnectionTest]
Removed now-unneeded DrillPrepareResult cast.
Eliminated redundant getConnection() and getClient calls, intermediate variables, etc.
Fixed comment error.
this closes #143
