Add send_packet to tds_socket in any cases

Use packet structure to store packet we are preparing to send.
This make symmetric the data for sending and receiving packets.
Changed code to reallocate socket to use tds_realloc_packet.

Signed-off-by: Frediano Ziglio <freddy77@gmail.com>
Remove pos from packet structure

pos field was used only during sending/receiving packet.
Move these field as fields of TDSCONNECTION.
This make packet structure a bit smaller.

Signed-off-by: Frediano Ziglio <freddy77@gmail.com>