Do symbolication requests in parallel
Remove old serial-request code
Longer timeout, more worker processes.
Update chromehangs code version
Add timing info for symbolication
Add `signature` code.

This takes a raw stack and simplifies it into something that can
be aggregated.
WIP for minimizing symbolication requests
Combine requests where possible.
Be smarter about requesting symbols.

Find all unique symbols first, then fetch them each once. This
reduces the number of symbolication requests by a lot, and speeds
things up dramatically.
Use the correct code path in S3
Exit non-zero if we fail to fetch some symbols.

We can detect non-zero exits and include more logs in the job
output.
Limit to nightly channel.

All the chromeHangs data is from nightly builds, so we can limit
the analysis to that channel.
Fix a couple of bugs in signature generation.

Missing comma messed up one of the regex pieces, and we want at
MOST 15 stack frames, not at LEAST.
Add some test cases for generating signatures
Aggregate chromehang stack info for dashboard.
Remove the "stack map" stuff
Combine stacks and get the top N.

Aggregate chromehang stack info and get the top MAX_ROWS by
frequency (number of pings), median duration, total duration.
Add "sentinel" logic and clean raw addresses.
Test a bunch of the signature functionality
Add the "extract common stacks" step.