Improve package download endpoint

[MythicManiac]

Premise

• Package download requests account for around 10-40% of the total request volume at any given time, meaning even relatively minor improvements can have measurable impact.
• The download event logging implementation is from a date before redis was used in the stack and as such it's still using the database for event rate limiting. This is less efficient than relying on redis would be.
• The download event logging implementation isn't particularly useful as it's effectively just a rate limited counter. A minimum requirement for it to become useful for analytical or presentation purposes is to retain timestamps with the events.

Summary of changes

This PR can be broken down to three main changes:

• It eliminates some pointless queries in the PackageDownloadView implementation
• It introduces a redis based rate limiting solution to download event logging (without removing the legacy method)
• It introduces a new time series download event logging implementation and feature flags to control which implementation(s) are used.

Test were also added for this area of code (which was previously untested).

Result

Ultimately the changes here should allow for slightly more efficient download handling and paves the way for time series applications of download metrics.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (9e6116f) 92.81% compared to head (7d87d78) 92.89%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #961 +/- ## ========================================== + Coverage 92.81% 92.89% +0.08% ========================================== Files 283 286 +3 Lines 8459 8478 +19 Branches 786 788 +2 ========================================== + Hits 7851 7876 +25 + Misses 501 497 -4 + Partials 107 105 -2 ```

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