[fix] fix the client crash when the transaction coordinator not found

[RobertIndie]

Motivation

If the broker doesn't enable the transaction coordinator, the TC client will still be created without returning any error. And when creating the transaction, it will crash here:

pulsar-client-go/pulsar/transaction_coordinator_client.go

Line 221 in a2d8a9f

return atomic.AddUint64(&tc.epoch, 1) % uint64(len(tc.cons))

This is because the tc.cons will be emtpy.

Modifications

• Add new error ErrTransactionCoordinatorNotEnabled
• Return error ErrTransactionCoordinatorNotEnabled if the broker doesn't enable transaction

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API: (yes / no)
• The schema: (yes / no / don't know)
• The default values of configurations: (yes / no)
• The wire protocol: (yes / no)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / GoDocs / not documented)
• If a feature is not applicable for documentation, explain why?
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation