[master < ]Cache user to speed up PullPlan

[andrejtonev]

[master < Task] PR

• Check, and update documentation if necessary
• Provide the full content or a guide for the final git message

To keep docs changelog up to date, one more thing to do:

• Write a release note here, including added/changed clauses
• Tag someone from docs team in the comments

A new FineGrainedAuthChecker is created each time a PullPlan is created. Here we called GetUser(username) which goes to disk and fetches the user data. Caching the user will help speed this up (since the user almost never changes).

[vpavicic]

@andrejtonev - release note please :)

[andrejtonev]

@vpavicic

Performance improvement when running queries with authorization turned on (with a user).
FineGrainedPermission would read from disk on every query execution. Now we cache the user's info and avoid going to disk until a new user is detected.
NOTE: Changes to the user's privileges will be ignored until next login.

[vpavicic]

@andrejtonev ok?

Users with fine-grained privileges will experience better performance when running
queries due to user information cashing. Any changes to the user privileges will
be ignored until the next login.

[andrejtonev]

@vpavicic
perfect