Add support for SQL isolation levels

[iand675]

This adds support for assorted SQL isolation levels to all of the standard SQL backends.

There is a slight breaking change for any dependent SQL backends in that the connBegin function now takes an optional isolation level argument. This is necessary because for MySQL the transaction isolation level must be specified before calling begin.

• Bumped the version number
• Documented new APIs with Haddock markup
• Added @since declarations to the Haddock

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[paul-rouse]

I don't feel very competent to approve this, especially for postgresql which I haven't used for years, but, that said, I've looked through the changes and they seem to be fine. If nobody else steps in with comments, I'll go ahead in a few days.

[iand675]

Hi @paul-rouse, any chance you're ready to merge this yet?

[bitemyapp]

I'll take a look at this as I'm an active Persistent + PostgreSQL user.

[paul-rouse]

Thanks @bitemyapp !

[bitemyapp]

This is a very neat PR. Thank you @iand675, good seeing you around too :)

[iand675]

@bitemyapp Glad to you like it! Any notion on when it'll go up on hackage? Waiting to merge the corresponding yesod PRs til this is all the way shipped.

[bitemyapp]

@iand675 I'll take a sweep of open PRs this weekend, see what can make it in.

[ProofOfKeags]

Quickly wanted to check in and see if this can go to hackage/stackage. Otherwise would you recommend pinning this hash?