plan: remove mutexes

[vmg]

Description

The mutexes that were guarding the 'stats' section of plans in vtgate
and vttablet were not being properly obeyed. A lot of the code was
accessing the stats without acquiring the mutex first, mostly because
there is no global consistency requirement between all the fields of the
stats. Because of this, remove the Mutexes from both plans and switch to
using Atomic operations to handle the stats fields. This reduces the
sizes of the plans on cache and improves the accuracy of our memory
usage metrics, since we do not have accurate size calculations for mutex
objects, as they're foreign to the project.

Checklist

• Should this PR be backported?
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving
• VReplication
• Cluster Management
• Build/CI
• VTAdmin

[harshit-gangal]

With this change there is no consistency between all the plan stats. May be that is not important. Just wanted to call that out.

[vmg]

@harshit-gangal: yes, that's explained in the commit message:

A lot of the code was accessing the stats without acquiring the mutex first, mostly because there is no global consistency requirement between all the fields of the stats

In practice, there was no consistency between plan stats before either, because most of the code that was reading the stats was doing so without acquiring the mutex first. That's why I thought this would be a good idea.