It was left over in zcash/librustzcash#317 from an earlier version that
used the `grpc` crate. The version imported into this repository had
been migrated to `zcash_client_backend 0.6` which switched to the
`tonic` crate for gRPC, which doesn't use `tls-api`.
This is a direct port of zcash/librustzcash#317, with minor tweaks to
account for the missing `std::error::Error` impls that the original PR
included, and removing the `src/init.rs` file left over from a previous
rebase of the PR.
Jack Grigg