Add logging to digest building and email delivery.

[tkaemming]

This adds debug logging in a few places that can result in digest building
being skipped or email delivery being prevented. These cases shouldn't be
representative of any errors, but may be the byproduct of unexpected
configurations.

This can help debugging issues such as #2569.

[codecov-io]

Current coverage is 82.92%

Merging #2569 into master will decrease coverage by -0.08% as of f8db125

@@            master   #2569   diff @@
======================================
  Files          861     861       
  Stmts        32406   32448    +42
  Branches         0       0       
  Methods          0               
======================================
+ Hit          26900   26909     +9
  Partial          0       0       
- Missed        5506    5539    +33

Review entire Coverage Diff as of f8db125

---

Uncovered Suggestions

1. +0.08% via ...try/utils/apidocs.py#414...436
2. +0.08% via ...try/utils/apidocs.py#117...139
3. +0.07% via ...gs/sentry_helpers.py#191...211
4. See 7 more...

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[tkaemming]

Example debug log output from digest generator:

2016-01-19 22:31:41,557 7639  DEBUG    sentry.digests <functools.partial object at 0x1056d9788> applied to 8 items.
2016-01-19 22:31:41,557 7639  DEBUG    sentry.digests <type 'bool'> filtered 8 items to 8.
2016-01-19 22:31:41,559 7639  DEBUG    sentry.digests <function check_group_state at 0x1056aeb90> filtered 8 items to 8.
2016-01-19 22:31:41,559 7639  DEBUG    sentry.digests <function group_records at 0x103caf758> reduced 8 items to 2.
2016-01-19 22:31:41,560 7639  DEBUG    sentry.digests <function sort_group_contents at 0x103caf7d0> applied to 2 items.
2016-01-19 22:31:41,560 7639  DEBUG    sentry.digests <function sort_rule_groups at 0x103caf848> applied to 2 items.