**Summary:**

1. _Primary_: Adds a `debug_only=true` flag for peertimeout, defaults to 60 sec., the current hard-coded setting.
2. _Secondary_: Drastically speeds up `p2p_timeout.py` test.
3. _Secondary_: Tests that the correct code path is being tested by adding log assertions to the test.

**Rationale:**

- P2P timeout was hard-coded: make it explicitly specified and configurable, instead of a magic number.
- Addresses #13518; `p2p_timeout.py` takes 4 sec. to run instead of 61 sec.
- Makes `p2p_timeout.py` more explicit. Previously, we relied on a comment to inform us of the timeout amount being tested. Now it is specified directly in the test via passing in the new arg; `-peertimeout=3`.
- Opens us up to testing more P2P scenarios; oftentimes slow tests are the reason we don't test.

**Locally verified changes:**

_With Proposed Change (4.7 sec.):_
```
$ time ./test/functional/p2p_timeouts.py
2018-11-19T00:04:19.077000Z TestFramework (INFO): Initializing test directory /tmp/testhja7g2n7
2018-11-19T00:04:23.479000Z TestFramework (INFO): Stopping nodes
2018-11-19T00:04:23.683000Z TestFramework (INFO): Cleaning up /tmp/testhja7g2n7 on exit
2018-11-19T00:04:23.683000Z TestFramework (INFO): Tests successful

real    0m4.743s
```

_Currently  on master (62.8 sec.):_
```
$ time ./test/functional/p2p_timeouts.py
2018-11-19T00:06:10.948000Z TestFramework (INFO): Initializing test directory /tmp/test6mo6k21h
2018-11-19T00:07:13.376000Z TestFramework (INFO): Stopping nodes
2018-11-19T00:07:13.631000Z TestFramework (INFO): Cleaning up /tmp/test6mo6k21h on exit
2018-11-19T00:07:13.631000Z TestFramework (INFO): Tests successful

real    1m2.836s
```

_Error message demonstrated for new argument `-peertimeout`:_
```
$ ./bitcoind -peertimeout=-5
...
Error: peertimeout cannot be configured with a negative value.
```

ninja check
test_runner.py
test_runner.py p2p_timeouts  # verify it completes in ~4 seconds