We don't add or maintain these, and they are of little value, as
well as having the effect of polluting diffs.
They are also wrong, i.e DEFAULT_SCRIPTCHECK_THREADS is not in
validation.h.
Needed for a later commit adding logic to ask the TxRequestTracker for a
list of announcers. These announcers should know the parents of the
transaction they announced.
This adds a new module (unused for now) which defines TxRequestTracker, a data
structure that maintains all information about transaction requests, and coordinates
requests.