Details of the issue this is trying to solve can be found in T194299

Task details

In T194299#4713654, @hoo wrote:

@aaron Suggested to use a separate master DB connection for this (if were in a transaction and don't immediately autocommit), so that it never ends up in the main transaction. That might mean slightly more wasted IDs, but should significantly reduce contention.

In T194299#4714614, @aaron wrote:

openConnection is badly named and still reuses connections. You'd probably want getConnection with CONN_TRX_AUTO

Acceptance criteria

• UpsertSqlIdGenerator can generate ids using a seperate master db connection
• SqlIdGenerator can generate ids using a seperate master db connection
• Use of a seperate master db connection is configurable
• The default configuration is false