Fix race condition in trace_api_plugin shutdown

[heifner]

Failure https://github.com/eosnetworkfoundation/mandel/runs/7143297502?check_suite_focus=true points to an issue in shutdown of the trace_api_plugin.

slice_directory::stop_maintenance_thread() sets atomic bool _maintenance_shutdown = true and calls notify_one() on the _maintenance_condition condition variable. This is a race condition because it doesn't acquire the condition variable mutex setting up the possibility that _maintenance_shutdown can be set to true and notify_one called after the while check in slice_directory::start_maintenance_thread but before the wait() causing it to wait forever. This then blocks the slice_directory::stop_maintenance_thread() call of join() on the main thread, blocking the shutdown of all other plugins.

debug 2022-07-01T03:23:35.304 nodeos    net_plugin.cpp:2705           update_chain_info    ] updating chain info lib 90, head 138, fork 138
info  2022-07-01T03:23:35.304 nodeos    resource_monitor_plugi:122    plugin_shutdown      ] shutdown...
debug 2022-07-01T03:23:35.304 net-0     net_plugin.cpp:3195           operator()           ] accepted signed_block : #138 23fb4fff5f2c0077...
debug 2022-07-01T03:23:35.304 net-1     net_plugin.cpp:2152           operator()           ] ["localhost:9876 - 465c8f8" - 3 127.0.0.1:9876] bcast block 138
debug 2022-07-01T03:23:35.304 net-1     net_plugin.cpp:2173           recv_block           ] ["localhost:9876 - 465c8f8" - 3 127.0.0.1:9876] canceling wait
debug 2022-07-01T03:23:35.304 net-1     net_plugin.cpp:1993           sync_recv_block      ] ["localhost:9876 - 465c8f8" - 3 127.0.0.1:9876] got block 138
info  2022-07-01T03:23:35.304 net-1     net_plugin.cpp:1016           _close               ] ["localhost:9876 - 465c8f8" - 3 127.0.0.1:9876] closing
info  2022-07-01T03:23:35.304 net-1     net_plugin.cpp:1227           operator()           ] ["localhost:9876 - 465c8f8" - 3 127.0.0.1:9876] async write socket closed before callback
info  2022-07-01T03:23:35.304 net-1     net_plugin.cpp:1016           _close               ] ["localhost:9876 - 465c8f8" - 3 127.0.0.1:9876] closing
info  2022-07-01T03:23:35.305 nodeos    resource_monitor_plugi:129    plugin_shutdown      ] exit shutdown
info  2022-07-01T03:23:35.305 nodeos    net_plugin.cpp:3772           plugin_shutdown      ] shutdown..
info  2022-07-01T03:23:35.305 nodeos    net_plugin.cpp:3789           plugin_shutdown      ] close 5 connections
debug 2022-07-01T03:23:35.305 nodeos    net_plugin.cpp:3792           plugin_shutdown      ] close: 1
debug 2022-07-01T03:23:35.305 nodeos    net_plugin.cpp:3792           plugin_shutdown      ] close: 2
debug 2022-07-01T03:23:35.305 nodeos    net_plugin.cpp:3792           plugin_shutdown      ] close: 3
debug 2022-07-01T03:23:35.305 nodeos    net_plugin.cpp:3792           plugin_shutdown      ] close: 4
debug 2022-07-01T03:23:35.305 nodeos    net_plugin.cpp:3792           plugin_shutdown      ] close: 5
info  2022-07-01T03:23:35.305 net-1     net_plugin.cpp:1016           _close               ] ["localhost:9876 - 465c8f8" - 1 127.0.0.1:53368] closing
info  2022-07-01T03:23:35.305 nodeos    net_plugin.cpp:3809           plugin_shutdown      ] exit shutdown
info  2022-07-01T03:23:35.305 nodeos    test_control_plugin.cp:139    plugin_shutdown      ] test_control_plugin shutting down
debug 2022-07-01T03:23:35.305 trace-mx  trace_api_plugin.cpp:155      operator()           ] Waking up to handle lib: 90