tcp: request tracking is not http specific.

This change is a name change only. TCP Request tracking can track sendfile and even non-sendfile requests. The
names however in the current code use http, and they should not. The feature is not http specific. Lets change the
name so they more properly reflect whats going on. This also fixes conflicts with http_req which caused application pain.

Reviewed by: tuexen
Sponsored by: Netflix Inc
Differential Revision:https://reviews.freebsd.org/D40229