RFC: Store blacklisted tables in vttablet instead of topo

[deepthi]

Requirement

As part of #4790, we have the following requirement:

For example, while migrating masters (or writes), we have to first force readonly on the source. This is currently achieved by pushing tablet control records or blacklisted tables into the topo. Instead, we’ll reimplement this by directly writing this metadata into the relevant vttablets where the action will be taken.

Proposed Design

• Add a table in _vt schema to hold the list of blacklisted tables. We will store a proto representation of the string array that represents this list.
• Add a new vttablet RPC UpdateBlacklistedTables
• Replace calls to ShardInfo.UpdateSourceBlacklistedTables with calls to TabletManagerClient.UpdateBlacklistedTables
• For now, we will leave the fields in topo, and deprecate them in a future release

[deepthi]

cc @sougou @rafael

[deepthi]

EDIT: When we get to this, we also need to replace blacklist with blocklist (or something similar).