Move JSON API controller transactions to middleware

[lindyhopchris]

In 1.x the JSON API controller wraps write operations in a transaction.

For 2.0, move this to an opt-in middleware, that takes the name of the connection and optionally the HTTP method names that it should apply to (which should default to POST, PATCH, DELETE if not provided).

Allow this to be added via fluent routing, e.g. $api->withTransactions()->routes(...).

The Eloquent adapter should always use a transaction, as it knows the connection etc for the transactions... so modify that adapter to always use transactions for write operations.

[lindyhopchris]

Closing this as it's not an issue with the new package.