tcp: config option for the read buffer size

tcp: config option for the read buffer size

- the read buffer size can now be configured both at runtime and
  from ser.cfg (tcp_rd_buf_size). A high value will help
  performance for tcp connections with lots of traffic, however it
  will increase the memory consumption. As a rule of thumb use
  high values(e.g. 32768, 65536) on servers which open only a few
  tcp connections and have very heavy traffic on them and a low
  value (e.g. 4096, 2048) on servers that are expected to have
  lots of open connections (50k - 100k+). Note also that this
  value will also limit the maximum sip datagram size that can be
  received on tcp. The default value is 4096.