## Problem
The communicator gRPC client must support changing the shard map on
splits.
Touches #11735.
Requires #12476.
## Summary of changes
* Wrap the shard set in a `ArcSwap` to allow swapping it out.
* Add a new `ShardSpec` parameter struct to pass validated shard info to
the client.
* Add `update_shards()` to change the shard set. In-flight requests are
allowed to complete using the old shards.
* Restructure `get_page` to use a stable view of the shard map, and
retry errors at the top (pre-split) level to pick up shard map changes.
* Also marks `tonic::Status::Internal` as non-retryable, so that we can
use it for client-side invariant checks without continually retrying
these.
## Problem
The rich gRPC Pageserver client needs to split GetPage batches that
straddle multiple shards.
Touches #11735.
Requires #12462.
## Summary of changes
Adds a `GetPageSplitter` which splits `GetPageRequest` that span
multiple shards, and then reassembles the responses. Dispatches
per-shard requests in parallel.
## Problem
For the communicator, we need a rich Pageserver gRPC client.
Touches #11735.
Requires #12434.
## Summary of changes
This patch adds an initial rich Pageserver gRPC client. It supports:
* Sharded tenants across multiple Pageservers.
* Pooling of connections, clients, and streams for efficient resource
use.
* Concurrent use by many callers.
* Internal handling of GetPage bidirectional streams, with pipelining
and error handling.
* Automatic retries.
* Observability.
The client is still under development. In particular, it needs GetPage
batch splitting, shard map updates, and performance optimization. This
will be addressed in follow-up PRs.
## Problem
The communicator will need gRPC channel/client/stream pools for
efficient reuse across many backends.
Touches #11735.
Requires #12396.
## Summary of changes
Adds three nested resource pools:
* `ChannelPool` for gRPC channels (i.e. TCP connections).
* `ClientPool` for gRPC clients (i.e. `page_api::Client`). Acquires
channels from `ChannelPool`.
* `StreamPool` for gRPC GetPage streams. Acquires clients from
`ClientPool`.
These are minimal functional implementations that will need further
improvements and performance optimization. However, the overall
structure is expected to be roughly final, so reviews should focus on
that.
The pools are not yet in use, but will form the foundation of a rich
gRPC Pageserver client used by the communicator (see #12462). This PR
also adds the initial crate scaffolding for that client.
See doc comments for details.
