access to `dolt_branches` is not safe in multi-tenant environment

Queries like `select * from dolt_branches` can fail under concurrent load when other clients delete a branch. This is because `dolt_branches` and some other system tables don't currently respect transactional isolation and always read from the current tip of the entire database.

Expectation is that branch information is managed transactionally, the same as table data.

First milestone is read isolation: `dolt_branches` should have the same contents as it did at transaction start throughout a transaction, regardless of changes made by other clients.

Second milestone is write isolation: prevent conflicting writes to a particular branch by multiple clients. We currently are only principled about moving branch heads transactionally for the `dolt_commit()` procedure. We need to bring in `dolt_branch()`, `dolt_reset()` and other operations that can move or delete a branch head.