Backend. Time series requests die due to dropped connection

[rfaulkner]

This happens when the connection is dropped and results in several hanging threads. One possible fix is to lower the thread count in metrics however, this is a kludge. Better handling around dropped connections, but also threads should be able to die gracefully. So this really addresses two issues:

1) Handle dropped connections up the stack trace so that worker processes can always die gracefully.

2) In cases where a metrics worker thread throws an exception the multiprocessing thread pool ends up hanging have some facility for detecting that and terminating those threads.

[rfaulkner]

it may be worth adding a context for these connections:

http://stackoverflow.com/questions/2837822/in-python-how-to-make-sure-database-connection-will-always-close-before-leaving

[rfaulkner]

https://github.com/rfaulkner/E3_analysis/commit/02cd02fad0fee970b6402e0c888df5b8a0084d30 https://github.com/rfaulkner/E3_analysis/commit/bc86f94095eddb4fc69048f9f39b4dbd65e85600 https://github.com/rfaulkner/E3_analysis/commit/d6ecc3a19b1dc9a11d65fff5546c7b2818c7ecd2