Proposal: Separate Env into Thread and FileSystem

[rockeet]

Expected behavior

Thread and FileSystem are independent components, when we implement a new FileSystem, we need not to implement Thread.

Actual behavior

Thread and FileSystem functions are wrapped by Env. If we just want to implement a new FileSystem, we also need to implement Thread.

I know we can use EnvWrapper or delegate to default env to mitigate this issue, but this may causing other issues:

1. Need access to default env
2. Increase unnecessary work efforts...

Steps to reproduce the behavior

Try to implement a new FileSystem.

[mrambacher]

I wholeheartedly agree with this suggestion.

There are many classes that attempt to implement parts of the functionality of an Env and attempt to implement/override one portion or another (like HdfsEnv and MockEnv do for FileSystem and others do for overriding timers. It would be much simpler if these interfaces would be split into separate classes.

[rockeet]

Now #5761 and #6552 have aimed on this proposal.

[mrambacher]

I think most of this work has been accomplished through the FileSystem and SystemClock classes.