collector's downloads: avoid httpurl-connection
In November 2012, tor26 was very slow in responding to metrics-db, which made metrics-db exceed its 30-minute schedule. This happened again on March 2, 2013. The workaround was to avoid tor26 and other slow directories and download descriptors only from the other directory authorities. metrics-db should use a timeout similar to how DocTor does.
This should avoid httpurl-connection as suggested in #20323 (moved) for metrics-lib.
This is the parent ticket for three smaller tasks.