dtls: use dynamic sizes for netq when possible

When each package stored in netq is dynamically allocated we should not
make space for DTLS_MAX_BUF, but allocate as many bytes as needed for
this specific packet. This reduces the space needed for the
retransmission queue by ~50% on a Linux system.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>