Crypto Performance Improvements

[lgrahl]

This PR intends to improve the overall encryption/decryption performance. It also adds a performance test suite.

Results from the new tests so far are very promising. The base (Main Thread (shared key store=false)) is what we used before this PR.

Results are from an AMD Ryzen 7 2700X (higher % is faster).

Encryption	Chrome 67.0.3396.99	Firefox 61.0.1
Main Thread (shared key store=false)	100% (base)	100% (base)
Main Thread (shared key store=true)	201%	281%
Web Worker (shared key store=false, transferables=false)	93%	95%
Web Worker (shared key store=false, transferables=true)	96%	97%
Web Worker (shared key store=true, transferables=false)	171%	239%
Web Worker (shared key store=true, transferables=true)	176%	245%

Decryption	Chrome 67.0.3396.99	Firefox 61.0.1
Main Thread (shared key store=false)	100% (base)	100% (base)
Main Thread (shared key store=true)	197%	275%
Web Worker (shared key store=false, transferables=false)	91%	93%
Web Worker (shared key store=false, transferables=true)	92%	96%
Web Worker (shared key store=true, transferables=false)	161%	235%
Web Worker (shared key store=true, transferables=true)	174%	254%

To Do:

• [x] Use SharedKeyStore by default in the client
• [x] Rebase against master
• [x] Add tests for SharedKeyStore
• [x] Update docs (need a local server for the performance tests)
• [x] Evaluate whether depending task implementations need to be updated to leverage SharedKeyStore (answer: no)

[dbrgn]

Also, before merging this, could you squash the two cleanup commits "Fix lint errors" and "Cleanup let -> const in keystore.spec.ts" into their preceding commits? They don't add any value as standalone commits :slightly_smiling_face:

[dbrgn]

I fixed the remaining issue. Will merge once tests pass.