openthread: platform: mdns_socket: Avoid assert in high-traffic networks

This commit implements a rule that will silently discard incoming mDNS
messages if number of OT free message buffers after message conversion
will be under a user-predefined threshold.
mDNS is making use of OT message buffers, if an mDNS TX message can't be
allocated, it will trigger an assert.

Signed-off-by: Cristian Bulacu <cristian.bulacu@nxp.com>