fix(ledger): prevent panic on snapshot manager shutdown

[Ady0333]

Type of change

• Bug fix

Description

This PR fixes a shutdown race in the ledger snapshot manager that could cause a peer to panic.

Snapshot generation runs asynchronously in goroutines. If a peer shuts down while a snapshot is still in progress, those goroutines could attempt to send events on the events channel after it has been closed in shutdown(), resulting in a send on closed channel panic.

The fix coordinates snapshot goroutines with shutdown so that no sends occur after shutdown begins.

Additional details

Snapshot goroutines are now tracked and are signaled to stop during shutdown.
shutdown() waits for in-flight snapshot work to finish before closing channels.

Tested locally with go test ./core/ledger/kvledger/...

Release Note

Fixes a shutdown race in the ledger snapshot manager that could cause a peer
panic when snapshots are generated during shutdown.

[Ady0333]

Hi @C0rWin @denyeart @yacovm,

This PR fixes a shutdown race in the snapshot manager where a snapshot goroutine could send on a closed channel and panic.
The change makes shutdown safe even if a snapshot is still in progress.

Thanks for taking a look.