*: Prepare release candidate 0.10.0-rc.1

[mxinden]

Following the basic release approach suggested in #305 (comment) to create a release candidate (0.10.0-rc.1).

Once this is merged I would:

1. Create and push a git tag.

   tag="v$(sed -En 's/^version = \"(.*)\"$/\1/p' Cargo.toml)"
   git tag -s "${tag}" -m "${tag}"
   git push tikv --tags --dry-run
   git push tikv --tags

2. Publish to crates.io.

   cargo clean
   git clean -fd
   cargo publish --dry-run
   cargo publish
   

I am still not against the more advanced release process for future releases suggested by @lucab in #305 (comment).

[mxinden]

Sorry for the force-push. I forgot to sign off my first commit.

@lucab would you mind taking another look?

@breeswish any objections?