Mesh reply counters (#1374)

Mesh reply counters (#1374)

* Statistics counter for number of queries dropped by limit on reply addresses

Request list entries can be associated with multiple pending "reply
addresses". Basically each request list entry keeps its own list of
clients that should receive the response once the recursion is finished.
This requires keeping allocations around for each client, and there is
a global limit on the number of *additional* reply addresses that can
be allocated. (Each new request list entry seems to get its own initial
reply address which is not counted against the limit.)

This commit adds a statistics counter "num_queries_replyaddr_limit" that
counts the number of incoming client queries that have been dropped due
to the restriction on allocating additional reply addresses. This allows
distinguishing these drops from other kinds of drops.

* Statistics counter for number of mesh reply entries

Request list entries can be associated with multiple pending "reply
addresses". Since there is a limit on the number of additional reply
addresses that can be allocated which can cause incoming queries to be
dropped if exceeded, it would be nice to be able to track this number.

This commit basically exports the mesh_area's internal counter
`num_reply_addrs` as "threadX.requestlist.current.replies" /
"total.requestlist.current.replies".