Support manual cache eviction

[StevenSmiley]

Issue #, if available: aws#12

Description of changes:

This change adds support for manual cache eviction upon POST request with a secret id.

• Modify the server to allow POST to the /evict path.
• Add a function to handle eviction requests.
• Update the cache manager to support removing a specific entry from the cache.
• Update relevant tests.

[StevenSmiley]

On second thought, do we actually need to allow this feature to be disabled through the config? I initially thought that would be nice to support, but now I think we should remove it for simplicity sake. I can't think of situations where it would be important to disable. Another alternative would be to leave the config but set the default to allow.

[StevenSmiley]

On second thought, do we actually need to allow this feature to be disabled through the config? I initially thought that would be nice to support, but now I think we should remove it for simplicity sake. I can't think of situations where it would be important to disable. Another alternative would be to leave the config but set the default to allow.

Decided to remove this config