[Improve] improve the perf of schema and schema cache

[gunli]

(If this PR fixes a github issue, please add Fixes #<xyz>.)

Fixes #

Master Issue: #

Motivation

SchemaInfo.hash() and schemaCache.Get() are frequently called by partitionProducer.internalSend(), current implementation of these functions use sync.RWMutex to sync the cache map and calculate the hash every call, use the sync.Map and cache the hash value can improve the performance.

Modifications

Describe the modifications you've done.

• Modified: pulsar/producer_partition.go
• Modified: pulsar/schema.go

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

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

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

This change added tests and can be verified as follows:

(example:)

• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

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): (no)
• The public API: (yes / no)
• The schema: (yes)
• The default values of configurations: (no)
• The wire protocol: ( no)

Documentation

• Does this pull request introduce a new feature? (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

[shibd]

Thanks for your contribution. Left some small comments.