[Test] Speed up functional tests

[panleone]

The aim of this PR is making functional test run faster

First commit:
Sleeping inside wait_for_rpc_connection is useless since in case of failure it will try again rather than throwing error.

Second commit:
profiling showed that waiting for all nodes to connect to each other was very slow (8 nodes took about ~32 seconds on my PC). If we connect them in parallel time is much faster ( more or less 4 seconds on my PC)

Third commit:
sleeping inside is_node_stopped is useless since it is always used in operations like wait_until_stopped, and in case a node is not stopped it will wait and try again rather than throwing error.

Fourth commit:
removed many other big time.sleep() that made everything go overall much slower. This is the only commit that I'm not 100% sure so if we see that some test fail at the beginning (I mean before run_test() is called) this one can be reverted.

Fifth commit:
Make faster and solve the failure of p2p_invalid_messages.py:
When receiving lots of messages use a timeout system, instead of sleeping for an arbitrary amount of time (30 seconds, which sometimes was not enough)

[Liquid369]

tACK 26ed4d4
Logically all these changes check out, I've reran the tests a few times on my own and they all have been successful.
Much needed improvements and very happy that we can speed up the test suite.

[Duddino]

utACK 26ed4d4

[Fuzzbawls]

ACK 26ed4d4

Nice speed up in the test suite