vtorc: move shard primary timestamp to time type

[timvaillancourt]

Description

This PR move the shard primary timestamp in VTOrc to be represented as a real time.Time, not just a string (that is currently only compared with != and ==)

While updating this, I noticed the .GetTime() method of rows in sqlutils is broken. It seems the way time.Time is represented as .String() changed since the time this code was written. We've been unaffected by this breakage because, before this PR, .GetTime() was called in a single place where we never use the value, so we don't know it's broken (an empty time.Time{}). See unit tests for more context

Related Issue(s)

Aides in implementation of #18337. Aside from this, having a time-comparable timestamp will aid in a future feature I would like to add

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.49%. Comparing base (fae2b51) to head (32f9a22).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18401      +/-   ##
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- Coverage   67.49%   67.49%   -0.01%     
==========================================
  Files        1603     1603              
  Lines      262495   262495              
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- Hits       177180   177170      -10     
- Misses      85315    85325      +10     

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[timvaillancourt]

There is a minor memory implication to this. In the current approach, and empty timestamp is stored as just "". In this new approach we're always storing a full timestamp, just a zero-ed out one if it's empty

If this is a concern I could use *time.Time instead of time.Time (and remove NOT NULL in schema), which will allow an empty/nil value to be stored as NULL. cc @GuptaManan100 / @deepthi for 💡s

[GuptaManan100]

I don't think VTOrc performance is that much of a concern. In our experiements we found it wasn't a CPU or memory bound workload. Mostly dependent on the network calls, so its fine I think

[timvaillancourt]

I don't think VTOrc performance is that much of a concern. In our experiements we found it wasn't a CPU or memory bound workload. Mostly dependent on the network calls, so its fine I think

@GuptaManan100 yeah, I agree. For us a /api/database-state dump of an instance with 3000~ tablets is still just a couple of MB, including the json indentation/overhead. I guess there are some *cache.Cache objects too, but the total memory usage should be just 1 to 10s of megabytes

I think the only resource we'd be concerned to increase is CPU or remote topo calls 👍