IMPALA-14131: Add flag to configure the default value of

'impala.disableHmsSync'

FEATURE: Implement global 'disable_hms_sync_by_default' flag for event
processing. This change introduces a new catalogd startup flag,
`disable_hms_sync_by_default`, to simplify skipping/processing events.

Problem: Disabling event processing globally requires tedious process
of setting 'impala.disableHmsSync' property on every database and table,
especially if few specific tables requires sync up of events.

Solution: The new flag provides a global default for the
'impala.disableHmsSync' property.

Behavior:
- If `disable_hms_sync_by_default` is true (the intended default-off
state), event processing is skipped for all tables/databases unless
the property "impala.disableHmsSync"="false" is explicitly set.
- This allows users to easily keep event processing off by default
and opt-in specific databases or tables to start syncing.
- The check order is: table-property > db-property > global default.
- HMS polling remains independent and unaffected by this flag.

Change-Id: I4ee617aed48575502d9cf5cf2cbea6ec897d6839
Reviewed-on: http://gerrit.cloudera.org:8080/23487
Reviewed-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>
Tested-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>

common/thrift/BackendGflags.thrift

@@ -361,4 +361,6 @@ struct TBackendGflags {
   165: required i32 min_jdbc_scan_cardinality
 
   166: required i32 max_stmt_metadata_loader_threads
+
+  167: required bool disable_hms_sync_by_default
 }