Memory leak in hackage-mirror

[edsko]

@kosmikus reported that hackage-mirror uses a lot of RAM on its first sync.

[d3dave]

On a 4GB RAM + 4GB swap machine, hackage-server run attempts to utilize so much ram that all the swap space + ram gets used and the machine becomes totally unresponsive.

Worse yet, this doesn't happen after running for a long time, but merely 15min. after the command is executed.