Refactor auth backend

[g-k]

Refactoring for #96

• Move the following autographer data structures:

signers       []signer.Signer
auths         map[string]authorization
signerIndex   map[string]int

to an in-memory implementation of a swappable authbackend interface that hides the signerIndex implementation details and update callers accordingly

• remove makeSignerIndex from the autographer and index as auths are added
• expose a getSignerForUser(userID, signerID string) (signer.Signer, error) method instead of getSignerID (which depends on access to the autographer signers and signerIndex)

r? @jvehent

[coveralls]

Pull Request Test Coverage Report for Build 4232

• 130 of 139 (93.53%) changed or added relevant lines in 5 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 74.129%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
main.go	19	20	95.0%
authorize.go	6	8	75.0%
memory_backend.go	89	95	93.68%

Files with Coverage Reduction	New Missed Lines	%
database/connect.go	3	70.83%

Totals
Change from base Build 4227:	0.2%
Covered Lines:	3470
Relevant Lines:	4681

---

💛 - Coveralls

[jvehent]

looks good. a few nits but nothing important.

[jvehent]

r+. The code looks much cleaner too, which is 👌

[g-k]

refs: #96