Support for off-loop read-ony qp-trie transactions

Support for off-loop read-ony qp-trie transactions

It is sometimes necessary to access a qp-trie outside an isc_loop,
such as in tests or an isc_work callback. The best option was to use
a `dns_qpmulti_write()` transaction, but that has overheads that are
not necessary for read-only access, such as committing a new version
of the trie even when nothing changed.

So this commit adds a `dns_qpmulti_read()` transaction, which is
nearly as lightweight as a query transaction, but it takes the mutex
like a write transaction.