Summary:
If read queries are executed inside a transaction block
with SERIALIZABLE READ ONLY DEFERRABLE, we wait for
max_clock_skew_usec in Tablet::DoGetSafeTime. We are missing
a wait event for this wait, this diff adds that.
Jira: DB-17445

Test Plan: ./yb_build.sh --cxx-test wait_states-itest --gtest_filter WaitStateITest/AshTestVerifyOccurrence.VerifyWaitStateEntered/kWaitForReadTime

Reviewers: amitanand, hbhanawat

Reviewed By: amitanand

Subscribers: ybase

Differential Revision: https://phorge.dev.yugabyte.com/D44983