[5.5] Allow for configuration of sticky databases

[laurencei]

Allow #20445 to be configurable

I can forsee edge cases where people might not want this functionality (i.e. doing a small write followed by unrelated large read in same request - such as an application that logs all user actions) - so we can easily make it configurable for those that want either behavior.

Also this way we preserve current behavior - because the config will be null on existing applications (since the value doesnt exist) - so this reduces risk of strange breaking behavior during a 5.5 upgrade.

Instead I propose to mention this new config option in the 5.5 upgrade guide, for those that want to enable it. And meanwhile I can do another PR for laravel/laravel to include it in the default config stack and in the docs.

I'll do the other PR's for you if this gets accepted.

[taylorotwell]

So it will default to false / null I guess? That's your intention?

[laurencei]

That's the idea. It means the current behaviour is kept, but we offer the ability/knowledge to change during the upgrade. My fear is issues being posted after 5.5 where people's prod environment suffers due to heavy reads on the write database (if they are doing both on the same cycle). This gives the best of both worlds. It could even be set to 'true' on Laravel/Laravel defaults - so new apps get the benefit. Just existing ones won't get hit...