Add fuzzing harness for ProcessMessage(...). Enables high-level fuzzing
of the P2P layer.

All code paths reachable from this fuzzer can be assumed to be reachable
for an untrusted peer.

Seeded from thin air (an empty corpus) this fuzzer reaches roughly 20
000 lines of code.

To test this PR:

$ make distclean
$ ./autogen.sh
$ CC=clang CXX=clang++ ./configure --enable-fuzz \
      --with-sanitizers=address,fuzzer,undefined
$ make
$ src/test/fuzz/process_message
…

Worth noting about this fuzzing harness:

    To achieve a reasonable number of executions per seconds the state
of the fuzzer is unfortunately not entirely reset between test_one_input
calls. The set-up (FuzzingSetup ctor) and tear-down (~FuzzingSetup) work
is simply too costly to be run on every iteration. There is a trade-off
to handle here between a.) achieving high executions/second and b.)
giving the fuzzer a totally blank slate for each call. Please let me
know if you have any suggestion on how to improve this situation while
maintaining >1000 executions/second.
    To achieve optimal results when using coverage-guided fuzzing I've
chosen to create one specialised fuzzing binary per message type
(process_message_addr, process_message_block, process_message_blocktxn ,
etc.) and one general fuzzing binary (process_message) which handles all
messages types. The latter general fuzzer can be seeded with inputs
generated by the former specialised fuzzers.

Happy fuzzing friends!

ninja bitcoin-fuzzers
./src/test/fuzz/process_message
./src/test/fuzz/process_message_getheaders # Or any other message