dgram: support for setting socket buffer size

* setRecvBufferSize(int) and setSendBufferSize(int)
* added docs for send/receive buffer sizes
* Added options support to set buffer sizes in
  dgram.createSocket().

PR-URL: https://github.com/nodejs/node/pull/13623


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>