Enables -Wall and -Werror in CIs (#254)

Removed warnings in:

* bench/echo_server_client.cpp (parameter shadowing)
* include/boost/redis/adapter/detail/adapters.hpp (unused parameter)
* include/boost/redis/connection.hpp (unused parameter)
* include/boost/redis/resp3/impl/parser.ipp (signed to unsigned conversion)
* test/test_conversions.cpp (signed to unsigned comparison)
* test/test_issue_50.cpp (superfluous move)
* test/test_low_level_sync_sans_io.cpp (signed to unsigned comparison)

tools/ci.py

@@ -295,6 +295,8 @@ def _run_b2_tests(
         'toolset={}'.format(toolset),
         'cxxstd={}'.format(cxxstd),
         'variant={}'.format(variant),
+        'warnings=extra',
+        'warnings-as-errors=on',
         '-j4',
         'libs/redis/test'
     ])