subscriber: feature-flag recording

[hawkw]

The recording features in console-subscriber requires additional dependencies (e.g. serde-json) that we don't need anywhere else in the console. My PR #238 will probably also add a crossbeam-channel dependency to the recording code. It would be kind of nice if the recording feature was feature-flagged so that these dependencies can be disabled by users who aren't using recordings.

The one potential concern about feature flagging this is that it could be considered a breaking change. However, feature-flagging it wouldn't make any APIs disappear, just behavior. Should this be considered a semver-breaking change?

[hawkw]

cc @seanmonstar