Just a preparatory commit to add the header to the includes and run
clang-format to sort the include lists.
Splitting this up into a separate commit makes future scripted-diffs
easier.
This adds a new RPC method "getzmqnotifications", which returns
information about all active ZMQ notification endpoints. This is useful
for software that layers on top of bitcoind, so it can verify that
ZeroMQ is enabled and also figure out where it should listen.
See https://github.com/bitcoin/bitcoin/issues/13526.
