Add ASIO completion handler tracking

[wezrule]

Boost asio has a way to output diagnostic information about completion handlers by defining BOOST_ASIO_ENABLE_HANDLER_TRACKING you get output like so:

@asio|1512254357.979980|0*1|signal_set@0x7ffeaaaa20d8.async_wait
@asio|1512254357.980127|0*2|socket@0x7ffeaaaa20f8.async_accept
@asio|1512254357.980150|.2|non_blocking_accept,ec=asio.system:11
@asio|1512254357.980162|0|resolver@0x7ffeaaaa1fd8.cancel
@asio|1512254368.457147|.2|non_blocking_accept,ec=system:0
@asio|1512254368.457193|>2|ec=system:0
@asio|1512254368.457219|2*3|socket@0x55cf39f0a238.async_receive
@asio|1512254368.457244|.3|non_blocking_recv,ec=system:0,bytes_transferred=141
@asio|1512254368.457275|2*4|socket@0x7ffeaaaa20f8.async_accept
@asio|1512254368.457293|.4|non_blocking_accept,ec=asio.system:11

Information about what these entries mean can be found here:
https://www.boost.org/doc/libs/1_72_0/doc/html/boost_asio/overview/core/handler_tracking.html

But they can also be turned into a graph: (described later)

By itself it's not that useful as we really care about how long these io thread completion handlers are taking. I've modified the thread_runner and system polling to now report any completion handlers which take longer than NANO_ASIO_HANDLER_TRACKING (new CMake variable). It outputs the timestamp which can be cross-referenced with the images/text to find what caused it. It is recommended to use less io_threads if trying to diagnose issues with these as multiple completion handlers cause be executing in parallel.

Proposed mode of operation:
1 - cmake -DNANO_ASIO_HANDLER_TRACKING=10 (report anything held for 10ms or greater)
2 - Lower io_threads in config-node.toml
3 - Run suspect code and pipe error output (asio uses that stream):
./core_test --gtest_filter=suspect_test 2> asio_output.txt
4 - For any "io_thread held for ??ms" messages, cross-reference with text and remove extraneous text surrounding it (see below)
5 - Generate image perl handlerviz.pl < asio_output.txt | dot -Tpng > output.png . If the asio_output.txt file is large this can crash or take a very long time, as well as make the image unreadable. dot binary is from GraphViz and can be installed for free on all platforms.

This can be a good first step in helping diagnose issues and inform of any completion handlers taking too long. A further step may be to wrap all completion handlers and break before exiting but that's a much larger effort with greater.