refactor: eliminate global storage_broker client state (#4318)

(This is prep work to make `Timeline::activate` infallible.)

This patch removes the global storage_broker client instance from the
pageserver codebase.

Instead, pageserver startup instantiates it and passes it down to the
`Timeline::activate` function, which in turn passes it to the
WalReceiver, which is the entity that actually uses it.

Patch series:

- #4316
- #4317
- #4318
- #4319

storage_broker/src/lib.rs

@@ -40,6 +40,9 @@ pub type BrokerClientChannel = BrokerServiceClient<Channel>;
 // Create connection object configured to run TLS if schema starts with https://
 // and plain text otherwise. Connection is lazy, only endpoint sanity is
 // validated here.
+//
+// NB: this function is not async, but still must be run on a tokio runtime thread
+// because that's a requirement of tonic_endpoint.connect_lazy()'s Channel::new call.
 pub fn connect<U>(endpoint: U, keepalive_interval: Duration) -> anyhow::Result<BrokerClientChannel>
 where
     U: std::convert::TryInto<Uri>,