Splitio-client

- Updated default auth service url to https://auth.split.io/api/v2/auth.
- Deprecated old telemetry.

- Added Cache-Control header for on-demand requests to sdk-server.
- Updated the synchronization flow to be more reliable in the event of an edge case generating delay in cache purge propagation, keeping the SDK cache properly synced.

- BREAKING CHANGE: Removed splitSdkMachineIp and splitSdkMachineName configs.
- BREAKING CHANGE: Deprecated `redisCharset` config.
- BREAKING CHANGE: Deprecated uWSGI local cache.
- BREAKING CHANGE: Deprecated Python2 support.
- Removed six, future and futures libs for compatibility between Python2 and Python3.
- Updated strings encoding to utf-8 by default for Redis.
- Added SDK Metadata headers to streaming client.

- Bumped mmh3cffi dependency which now requires c99 flag to build.

- Added RecordStats for supporting pipelined recording in redis when treatment call is made.
- Added hooks support for preforked servers.

- Fixed error handling when split server fails, so that it doesn't bring streaming down.
- Added SDK Metadata headers to split & segments API clients