fixing mixed value encodings

- adding test showing that mixed encodings per put worked already
- adding test to show that mixed encodings per sub did not work
- adding tests to show that readStream is not respecting encoding at
	all, but always emits strings

- adapting API to work with options in index and sub
- adapting all tests to pass in options instead of a string

- propagating sub encoding settings to make per sub encoding tests pass
- making readstream respect encoding by selectively merging passed opts
	onto the ones that are passed down to underlying db readStream
- all encoding tests pass at this point
fixing key and value streams

- adding tests that show that value and key streams always emit entire key/value object
- making value streams emit values only
- making key streams emit (fully namespaced) keys only
propagating additional options values to levelup
fixing mixed value encodings and merging key value streams fixes
depreciation message for sep
4.6.9