[oceanbase] Use global timestamp to start log client

[whhe]

This pr changed the startup order of snapshot and change events reading, and it will reduce the risk of change events buffer OOM.

[leonardBang]

Thanks @whhe for the contribution, LGTM, feel free to merge

[xinwen-cs]

In oceanbase-ce:3.1.4, SELECT TS_VALUE FROM oceanbase.V$OB_TIMESTAMP_SERVICE return microsecond rather than nanosecond

In oceanbase-ce:4.0.0.0, oceanbase.V$OB_TIMESTAMP_SERVICE or SYS.V$OB_TIMESTAMP_SERVICE does not exist

Is this code only test on enterprise edition?

[xinwen-cs]

@whhe

[whhe]

In oceanbase-ce:3.1.4, SELECT TS_VALUE FROM oceanbase.V$OB_TIMESTAMP_SERVICE return microsecond rather than nanosecond

In oceanbase-ce:4.0.0.0, oceanbase.V$OB_TIMESTAMP_SERVICE or SYS.V$OB_TIMESTAMP_SERVICE does not exist

Is this code only test on enterprise edition?

Thanks for your reply. The target version is 4.2.1, and you are right that 3.x and 4.x use different precision in clock value. I will add a fallback process for it as soon as possible.