Feature: Branch-level locking

A Hosted Dolt customer requested the ability to manually lock and unlock an entire branch from edits. This allows applications to restrict all updates to a branch while the lock is held, until it is released. We've seen this request on Discord, too.

As a workaround, many applications that need this can instead use [Dolt's existing support for named locks](https://dev.mysql.com/doc/refman/8.0/en/locking-functions.html#function_get-lock) – they don't provide update protection at the DB-level, but they can provide protection at the application-level. In other words, if all updates are going through the application code, then named locks can be a good workaround.

The issue names no files, tests, or entry points. Start by reviewing Dolt's existing named-lock support and branch update behavior, then clarify the public lock/unlock interface and acceptance checks for blocking edits until release.

Written by the indexing model from the issue text.