allow reading from a disconnected read-only slave

[ttlanhil]

Pull Request check-list

• [ ] Does npm test pass with this change (including linting)? The list of failing tests is unchanged / pull request does not cause new failures
• [ ] Is the new or changed code fully tested? Tested, but not covered by test files
• [x] Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

Description of change

My company has local read-only redis slaves that are allowed to cache stale data; so we needed a new option to "allow_disconnected_slaves": If set to true and the redis server is configured as slave-read-only then connections to a disconnected slave are allowed. Otherwise connections will be retried until the master connection is restored.