Change transaction request logic to use txrequest

This removes most transaction request logic from net_processing, and
replaces it with calls to a global TxRequestTracker object.

The major changes are:

* Announcements from outbound (and whitelisted) peers are now always
  preferred over those from inbound peers. This used to be the case for the
  first request (by delaying the first request from inbound peers), and
  a bias afters. The 2s delay for requests from inbound peers still exists,
  but after that, if viable outbound peers remain for any given transaction,
  they will always be tried first.

* No more hard cap of 100 in flight transactions per peer, as there is less
  need for it (memory usage is linear in the number of announcements, but
  independent from the number in flight, and CPU usage isn't affected by it).
  Furthermore, if only one peer announces a transaction, and it has over 100
  in flight and requestable already, we still want to request it from them.
  The cap is replaced with an additional 2s delay (possibly combined with the
  existing 2s delays for inbound connections, and for txid peers when wtxid
  peers are available).

Includes functional tests written by Marco Falke and Antoine Riard.

Author: sipa