Socket timeout tidyup

[dsiganos]

Tidy up of how timeouts are handled in socket class.

• Use guard ifs to make nano::socket::async_write more readable
• Removed needless code
• Added comments to nano::socket class
• Renamed the nano::socket concept of deadline to timeout
  A deadline makes people imagine a fixed point in time, a timestamp.
  However, socket::next_deadline is a timeout value and not a deadline.
  It signifies the seconds of inactivity that would cause a socket timeout.
• Remove network constant socket_dev_idle_timeout
• Rename socket::stop_timer to socket:set_last_completion
  All stop timer does is to set a variable. It is a simply setter function.
  It is confusing to call it stop_timer because it implies that it does
  something more than just set a timestamp.
• Added some socket_timeout unit tests

[thsfs]

It seems there is no major change other than tidying it up. They seem to be ok and the comments will be very helpful. @dsiganos, if you want to remove one more thing I could point the function socket::set_silent_connection_tolerance_time (std::chrono::seconds tolerance_time_a);. I added it to facilitate injecting a tolerance time for testing, also because it'd rather read if from the network_params, but it ended up unused.