HDDS-8166. [Snapshot] Add a config to enable or disable Ozone snapshot feature on OM

[smengcl]

What changes were proposed in this pull request?

Add an option to control whether Ozone filesystem snapshot feature is enabled or not on the OM side.

1. Add this new OM config.
2. Enable Ozone (filesystem) snapshot feature by default to avoid disruption with on-going dev work.
3. When disabled:
   a) All snapshot related OM background tasks shall not run
   b) Snapshot requests (CreateSnapshot / DeleteSnapshot) are gracefully rejected (OMException with FEATURE_NOT_ENABLED), achieved by a new AspectJ annotation implementation.
4. All existing and upcoming integration tests, UTs and acceptance tests related to Ozone snapshot feature would have a line to explicitly enable the snapshot feature for testing so those tests would still run and pass even when the default is switched to false.

There is a follow-up JIRA to refactor OmSnapshotManager constructor to further reduce the initialization done when snapshot feature is disabled: HDDS-8529.

TODO

• Remove extra omSnapshotManager null checks added in this PR now that we can't really not initialize it.

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/HDDS-8166

How was this patch tested?

• All existing tests shall pass.
• All existing tests shall pass even when default is set to false
  • https://github.com/smengcl/hadoop-ozone/commits/HDDS-8166-snapshot-config-false-4
• Add new UT to test the AspectJ annotation.
• Add a simple integration test to validate that client indeed get FEATURE_NOT_FOUND when the snapshot feature is disabled. Similar idea to HDDS-7772 but not quite the same implementation.

[prashantpogde]

LGTM

[errose28]

LGTM. @smengcl @prashantpogde can you file a follow up jira to define the long term story around this config flag? We need to decide if moving it from on to off and back to on is supported, and add corresponding tests or docs for whatever decision is made.

[smengcl]

LGTM. @smengcl @prashantpogde can you file a follow up jira to define the long term story around this config flag? We need to decide if moving it from on to off and back to on is supported, and add corresponding tests or docs for whatever decision is made.

Thanks @errose28 . I filed HDDS-8555 as a follow-up.

[smengcl]

Thanks @hemantk-12 @prashantpogde @errose28 for reviewing this.

[arp7]

We may not want to invest too much time in a long-term story, this flag is just for short-term use until the snapshot feature is fully ready and stable.