buildStages() STAGE_SHARDING_FILTER block should hold child stage tree by unique_ptr

When building a ShardFilterStage we will do the following:

1) Build the child stage tree, and hold by raw pointer

2) Get a CollectionShardingState object and use it to retrieve an orphans filter via call to CollectionShardingState::getOrphansFilter().

3) Create and return the ShardFilterStage

The call to CollectionShardingState::get() can dassert and the call to CollectionShardingState::getOrphansFilter() can uassert. In either case we will leak the child stage tree.

We should change this block to hold the child stage tree by unique_ptr and not release until after acquiring the orphans filter.