Allow for more control over dependencies

[rukai]

Hi I'm trying to improve incremental compilation times of my application.
I can see that couchbase is pulling in a lot of dependencies that are included in my application but not actually used by it.

Currently couchbase pulls in the following transitive dependencies that I would like to see made optional:

• hickory-resolver - this is a really large dependency for an optional feature, would be good to put behind a feature flag
• tracing-subscriber - tracing-subscriber is generally a dependency for applications only, so whatever functionality relies on it could probably go behind a feature flag
• hdrhistogram - another fairly big dependency, the logging_meter stuff looks like it could easily go behind a feature flag.

You can still keep the library easy to use by including all of these features in a default feature flag.
And you can use a tool like cargo hack in your CI to ensure that all combinations of features can build correctly.

I'm also not sure of your MSRV situation but you could get a small but easy win from replacing lazy_static with the std library alternatives.

Thanks!

[chvck]

Hi @rukai thanks for raising this, this seems like a reasonable request to me. I've raised https://jira.issues.couchbase.com/browse/RSCBC-275 for internal tracking purposes.