Cleanup sock and pid files on graceful exit from FastCGI

Summary: Notify the HTTPServer that we are terminating so that it will remove any pid
file, additionally cleanup unix socket file if necessary.

Fixes #3327

Reviewed By: @bertmaher

Differential Revision: D1672110

Signature: t1:1672110:1415903725:5ea192140219b9e1bc3c32a2c5ed978006987f86
Remove bad assert from BaseVector::data()

Summary: BaseVector::data() had an assert that was checking if m_data[0] was a Cell.
This assert is bogus because the Vector could be empty, and in such cases
it is not valid to dereference m_data[0]. It turns out that this assert was
redundant anywhere because the only place that uses BaseVector::data()
already asserts that each element in the Vector is a Cell (also, there are
a bunch of asserts throughout ext_collections that make sure that each
element is a Cell).

Reviewed By: @elgenie

Differential Revision: D1680393

Signature: t1:1680393:1415922724:0331eb4ed100231a24517cf4870736a07f56e275