kvserver: decouple lease requests from client context

[erikgrinaker]

Currently, lease requests (acquisitions and extensions) will be cancelled once all waiters go away:

https://github.com/cockroachdb/cockroach/blob/84a19f6e8f0c33b637deaa047c524ea6ab58f9a1/pkg/kv/kvserver/replica_range_lease.go#L105-L121

This is unfortunate in itself, because it might cause ranges to remain without leases indefinitely if lease acquisition takes a long time (e.g. because the previous leaseholder is unresponsive) and the client has a timeout that fires before the lease acquisition completes.

However, it is particularly problematic for expiration based leases, where we attempt to extend the lease asynchronously when a request comes in during the last half of the lease interval. This lease extension uses the client's context, but the client request is likely to complete before the lease extension does, which will cancel the lease extension. We already have a TODO saying as much:

https://github.com/cockroachdb/cockroach/blob/84a19f6e8f0c33b637deaa047c524ea6ab58f9a1/pkg/kv/kvserver/replica_range_lease.go#L1461-L1467

This problem is further exacerbated by the fact that concurrent requests won't join that pending lease request (which might otherwise have tided it over):

https://github.com/cockroachdb/cockroach/blob/84a19f6e8f0c33b637deaa047c524ea6ab58f9a1/pkg/kv/kvserver/replica_range_lease.go#L1434-L1436

Jira issue: CRDB-25277

Epic CRDB-25206