fix race condition between bootstrap pull and push

[dsiganos]

The unit test bootstrap_processor.push_one fails intermittently and it
exposes a race condition in the bootstrap code of the node. #3533

The race condition is that after finishing with bootstrap frontiers pull,
the code is supposed to put the socket back into the idle bootstrap
connections queue. However it does that in a racy way. The problem is in
the function nano::frontier_req_client::received_frontier.

Near the bottom of that function, we should call:
connection->connections.pool_connection (connection);
before calling
promise.set_value (false);

Once the promise is given a value then
nano::bootstrap_attempt_legacy::run()
continues to call push_request in parallel with pool_connection and it is
a race condition.

Although this is a bug, this is likely not a major nor catastrophic bug
because the bootstrap push functionality is only an efficiency.

resolves #3533
resolves #3720

[theohax]

2022-02-08T04:32:05.3477625Z + [[ Linux == \L\i\n\u\x ]]
2022-02-08T04:32:05.3477883Z + clang --version
2022-02-08T04:32:05.3478439Z ./ci/build-travis.sh: line 33: clang: command not found
2022-02-08T04:32:05.3478947Z + BACKTRACE=-DNANO_STACKTRACE_BACKTRACE=ON
2022-02-08T04:32:05.3480243Z + cmake '-GUnix Makefiles' -DACTIVE_NETWORK=nano_dev_network -DNANO_TEST=ON -DNANO_GUI=ON -DPORTABLE=1 -DNANO_WARN_TO_ERR=ON -DCMAKE_BUILD_TYPE=Debug -DCMAKE_VERBOSE_MAKEFILE=ON -DBOOST_ROOT=/tmp/boost -DNANO_SHARED_BOOST=ON -DQt5_DIR=/usr/lib/x86_64-linux-gnu/cmake/Qt5 -DCI_TEST=1 -DNANO_STACKTRACE_BACKTRACE=ON ..
2022-02-08T04:32:05.3481095Z fatal: No names found, cannot describe anything.
2022-02-08T04:32:05.3482385Z CMake Warning at flatbuffers/CMake/Version.cmake:22 (message):
2022-02-08T04:32:05.3599182Z   git describe failed with exit code: 128
2022-02-08T04:32:05.3599477Z Call Stack (most recent call first):
2022-02-08T04:32:05.3599737Z   flatbuffers/CMakeLists.txt:559 (include)
2022-02-08T04:32:05.3600146Z 

Probably this makes the CI fail, I'll have a look into it. Fix itself looks good to me (y)

Oh, actually I think this is the culprit:

[ RUN      ] block_store.empty_bootstrap
+ core_test_res=124

Looks like a timeout?

[dsiganos]

Return value 124 is the timeout code of the command timeout.
The test block_store.empty_bootstrap hang.

[theohax]

Return value 124 is the timeout code of the command timeout. The test block_store.empty_bootstrap hang.

Yes, looks like a timeout... probably a buggy test therefore