OT_TCP_ENDPOINT_TCB_SIZE_BASE macro

OT_TCP_ENDPOINT_TCB_SIZE_BASE and OT_TCP_ENDPOINT_TCB_NUM_POINTERS are chosen such that the mTcb field of otTcpEndpoint has the same size as struct tcpcb in TCPlp. This is necessary because the mTcb field, although opaque in its declaration, is treated as struct tcpcb in the TCP implementation.