Summary:
Read inside serializable transaction in the `WarmupTablespaceCache` helper function introduces extra delay in `asan`/`tsan` in several unit test (these tests add additional delay in distributed txn for testing purposes via GFlags). As a result units test are executed for a long time and also may fail due to timing issue.

The fix is to make a read from default transaction (read committed or repeatable read) which doesn't use distributed transaction for read operations.
Jira: DB-18960

Test Plan: Jenkins

Reviewers: esheng, bkolagani, #db-approvers

Reviewed By: bkolagani, #db-approvers

Subscribers: svc_phabricator, yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D48049