CSI plugin counts can drift from the correct values if allocations for a CSI
plugin stop concurrently, as with a job stop or a controller and node plugin
on the same client. This test demonstrates the behavior.

CSI volumes need to be "denormalized" with their plugins and (optionally)
allocations. Read-only RPC endpoints should take a snapshot so that we can
make multiple state store method calls with a consistent view.

When making updates to CSI plugins, the state store methods that have open
write transactions were querying the state store using the same methods used
by the CSI RPC endpoint, but these method creates their own top-level read
transactions. During concurrent plugin updates (as happens when a plugin job
is stopped), this can cause write skew in the plugin counts.

Refactor the CSIPlugin query methods to have an implementation method that
accepts a transaction, which can be called with either a read txn or a write
txn.

Add missing nil checks and missing watch `Add` calls on some state store
methods for CSI volumes

When making updates to CSI volumes, the state store methods that have open
write transactions were querying the state store using the same methods used
by the CSI RPC endpoint, but these method creates their own top-level read
transactions. These have yet not been implicated in any bugs.

Refactor the CSIVolume query methods to have an implementation method that
accepts a transaction, which can be called with either a read txn or a write
txn.