Bump the minimal libuv version to 1.34.0

By the time BIND 9.20 is release, all distributions without libuv >= 1.34.0 will be either EOL or on life-support. But even with the old distributions, the libuv could be updated from backports (or by installing it locally).

Bumping the minimal libuv version to 1.34.0 allows us to drop all the libuv shims that we've been maintaining.