[improve] return ErrMaxConcurrentOpsReached when too many concurrent ops in transaction coordinator client

[RobertIndie]

Motivation

Currently, the client will return an UnknownError when there are too many concurrent ops in transaction coordinator client

Modifications

• Add new error ErrMaxConcurrentOpsReached
• Return ErrMaxConcurrentOpsReached when too many concurrent ops in transaction coordinator client

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

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

[BewareMyPower]

Please fix the conflicts with master