The query shard reset() method resets some internal state in the
query shard context, like clearing query names, the filter flag
or named queries. The problem with this method being public is
that it currently (miss?) used for modifying an existing context
for recursive invocatiob, but the contexts that have been reseted
that way cannot be properly set back to their previous state.

This PR is a step towards removing reset() entirely by first making
it only be used internally in QueryShardContext. In places where
reset() was used we can either create new QueryShardContexts or
modify the existing context because it is discarded afterwards anyway.