Update boost::asio usage to conform to newer standards:

[vtnerd]

• Convert boost::asio::io_service to boost::asio::io_context
• Drop usage of GET_IO_SERVICE macro from monero
• Refactor REST server to manage resources better

[vtnerd]

Force pushed two more widespread updates:

• Convert strand.wrap(...) to boost::asio::bind_executor(strand, ...)
• Convert strand.dispatch(...) to boost::asio::dispatch(strand, ...)

There is one exception of strand.wrap(...) usage in src/rest_server.cpp because boost::bind_executor doesn't provide exact same functionality.

[vtnerd]

Force pushed some more updates:

• Convert io_context.reset() to io_context.restart()

[vtnerd]

Force pushed a rebase so that the newest io_context.reset() calls are now io_context.restart().

[vtnerd]

Forgot to include latest changes, trying another force push.

[vtnerd]

Rebased after latest bug fixes (there were merge conflicts).

[vtnerd]

Discovered another deprecated ASIO feature in use:

• Convert null_buffers() usage to socket.async_wait(...)

The changes have been force pushed.