[HYRAX-1732] Add SignedUrlCache and cache contents of s3credential endpoints

[hannahilea]

Description

• Cache contents of s3credential endpoint urls
• Set up and test SignedUrlCache class, as alternative to EffectiveUrlCache

This is a big step toward HYRAX-1732, but does not yet implement the signing itself---end-users will still receive effective urls created via service chaining. This PR adds the main plumbing to set us up for self-signing in the next PR. Includes the self-signing; will close HYRAX-1732 when tested/merged.

Tasks

• Ticket exists and is linked in title
• Tests added/updated
• Dead code removed
• No TODOs added (well, one! but intentionally b/c that HAS to happen next)
• Linking works on all Travis builds
• End-to-end tests added (and run on travis)
• Tested on SIT

[hannahilea]

Opening this for review but it should NOT be merged until the next PR is ready to go (and tested on SIT)---don't want to add unnecessary steps to the service chain until it actually buys us the win!

[hannahilea]

There is a fair amount of code added; probably the easiest way to understand it is to start by reading through the test page, since that is fairly comprehensive (I think).