endpoint: set destination_endpoint for created requests
endpoint: remove unreferenced static method
endpoint: fix incorrect semantics for is_same_host

The algorithm in CoAP 6.4.5 permits eliding Uri-Host only if the
hostname from the URI is an IP literal matching the endpoint.  DNS
lookup must not be done.

Also correct mis-use of getaddrinfo flags: AI_NUMERICSERV means the
service should not be looked up by name if it's already an integer; keep
that flag and verify that port is always an int.  AI_NUMERICHOST means
the node must already be in numeric format; this may not be the case
when endpoints are created, so remove the flag.
option: support stringize of unrecognized options
message: do message content validation and endpoint optimization
util: add TimeDueOrdinal support
message: core infrastructure supporting message ID caches
message: refine message ID cache infrastructure

Cache entries need to be permanently associated with the cache, and
updates to their time_due attribute should cause their position in the
cache queue to change.  Eliminate all interface to add cache entries
post-construction, and support only one interface to remove a cache
entry (pop_oldest).