feat: make encode/decode support store-based session

[Runrioter]

• Make encode/decode support store-based session.
• Need not add new options to support feat: support options.externalKey #88
• Modify test from generator function to async function

[coveralls]

Coverage remained the same at 100.0% when pulling d248017 on Runrioter:master into c040b59 on koajs:master.

[Runrioter]

@dead-horse hi, do you have any time to review this PR. This feature is necessary for me. 😃

[popomore]

Do you have read https://github.com/koajs/session/blob/master/Readme.md#external-session-stores

[Runrioter]

@popomore Yay I did. But sometimes the externalKey from cookie is not a key for a session, and we should encode(encrypt)/decode(decrypt) it.

Did you mean that I can wrap the encode/decode in external store get and set?
If I do that:

• I have to make a new repository like koajs/koa-redis instead of reusing it.
• The created external store just is used for session fetching and I have to wrap another layer for other user cases.
• It would not be the same as cookie-based session that has encode/decode func in this repo

Your opinion?