Detect resolver fetch loops only when joining an in-flight fetch

Detect resolver fetch loops only when joining an in-flight fetch

dns_resolver_createfetch() guarded against fetch loops by comparing the
raw request name/type/domain before any fetch context existed. Move the
check after the context is obtained and run it against the context
itself, and only when we joined an already in-flight context
(!new_fctx) that is also an ancestor in the parent chain. That is the
real loop condition: the new fetch would block waiting on a fetch that
is itself waiting on us. A newly created context waits on nothing, so it
proceeds, bounded by the fetch depth limit and the complementary ADB
loop detection.

(cherry picked from commit 0e04671b6508f9ff3cec85114300c136888a3852)