refactor(tvix/castore/tonic): make async, support wait-connect=?

This moves the sync `channel::from_url` to a async
`tonic::channel_from_url`. It now allows connecting non-lazily if `wait-
connect=1` is set in the URL params.

Also, make the pingpong tests for blobsvc and directorysvc use the wait-
connect=1 codepath.

Change-Id: Ibeea33117c8121814627e7f6aba0e943ae2e92ca
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10030
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/castore/src/lib.rs

@@ -2,11 +2,11 @@ mod digests;
 mod errors;
 
 pub mod blobservice;
-pub mod channel;
 pub mod directoryservice;
 pub mod fixtures;
 pub mod import;
 pub mod proto;
+pub mod tonic;
 pub mod utils;
 
 pub use digests::{B3Digest, B3_LEN};