CaptiveCoreBackend.isClosed can be confused with isPrepared

[bartekn]

What version are you using?

2.4.1

What did you do?

While working on #3694 I noticed that CaptiveCoreBackend.isClosed is confused with CaptiveCoreBackend.isPrepared. When CaptiveCoreBackend is closed by Close() call it can't be reused because internal ctx is cancelled and ledgerHashStore is closed however we often check isClosed() and the print an error saying: session is closed, call PrepareRange first.

What did you see instead?

I think what we should do is perform two checks:

• Check isClosed() - then prevent from using the CaptiveCoreBackend - PrepareRange and GetLedger should error.
• Check isPrepared() - if it's not return an error asking to prepare the range again.