This is the port shared between all application processes which use it to pass
requests for processing.  Using it significantly simplifies the request
processing code in the router.  The drawback is 2 more file descriptors per each
configured application and more complex libunit message wait/read code.

- Changed the port management callbacks to notifications, which e. g. avoids
the need to call the libunit function
- Added context and library instance reference counts for a safer resource
release
- Added the router main port initialization

According to libuv documentation, uv_poll_t memory should be released
in a callback function passed to uv_close().  Otherwise, the Node.js application
process may crash at exit.

This is required for Express framework compatibility.

This closes #418 issue on GitHub.

ServerResponse.write() method tries to write data buffer using libunit
and stores buffers to write in a Server-wide output queue, which is
processed in response to SHM_ACK message from router.

As a side effect 'drain' event implemented and socket.writable flag
reflect current state.

Name and value in each header are 0-terminated, so additional 2 bytes
should be allocated for them.  There were several attempts to add these
2 bytes to headers in language modules, but some modules weren't updated.
Also, adding these 2 bytes is specific to the implementation which may be
changed later, so extending this mechanics to modules may cause errors.

In Node.js version 11.10.0 and later, the writeHead() function returns "this".