* refactor: encapsulate request tracking in a class
Introduces a new class to peer-mgr, `ClientRequests`, which tracks what
active requests we've got pending: which blocks, when the requests were
sent, and who they were sent to.
This shouldn't change peer-mgr behavior. Its goal is to carve out some
of peer-mgr's data structures and encapsulte them behind an API that's
simpler to understand.
* refactor: move ActiveRequests to its own file
* perf: avoid duplicate call to tr_cpMissingBlocksInPiece
