* refactor: remove effective-url caching in webseeds
The upcoming CURLSH refactor is a more effective way of doing this and
the current appoach makes tr_web harder to refactor, so remove it.
* refactor: webseed rate limiting
The current code that limits the number of parallel fetches is overly
complicated and is also interwoven into the tr_webseed class. Extract
it into a new private helper class "ConnectionLimiter" with a simpler
public API for tr_webseed to use.
* 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
Charles Kerr