Add Error Limiting to slow nodes.

The proxy servers keep track of all requests and errors made to each node in a
shared SQLite database. Errors are defined as requests that resulted in a
timeout.  This data will be checked by the proxy before it sends future
requests to those nodes. It uses the existing error limiting functionality to
skip these nodes, however it will skip a maximum of one node based on this new
data. The SQLite databases should be stored ona tmpfs for speed and they are
rotated and removed occasionally, every 12 hours by default.

Change-Id: I37a63f209c989f72ef3aaa59934e6ec720911dce