[0.5d INTERRUPTION] Make push/pull compression optional

[alaric-dotmesh]

User requirement

As a dotmesh dev, I want to know if compression on push/pull is a net win or loss in some situations, so I can get the best performance for my users.

Key acceptance criteria

• [ ] It should be possible to disable compression when initiating a push or pull. A global environment variable is sufficient for this.
• [ ] Compressed and uncompressed pushes/pulls should both be accepted by the recipient of the push/pull, to maintain compatability and make it easy to do experiments: you only need to configure it on the initiator.
• [ ] To get a quick comparison against the current behaviour, the default should be that compression is OFF

[alaric-dotmesh]

Further research has confirmed that compression is a problem in our test env, so we're turning it off for now without an env var to turn it back on. Changing this is an easy fix if required, see the call to gzip.NewWriterLevel in dotmesh:pkg/utils/io.go.

[alaric-dotmesh]

This seems to be passing tests, so I can call it a wrap I think. The change is too trivial to bother with code review.