Support nested transactions using SAVEPOINT

[Jud]

The base branch for #111 was deleted, so I'm opening a new PR with a new base branch.

(Fixes #56)

[bjornpost]

That's quite a PR, nice work! I'll have a look at this once we have 1.3.2 released. Would you mind writing some documentation for this as well? IIRC transactions are currently not explained at all in the docs.

[bjornpost]

Code LGTM. An example of how to use in the docs would be great. While you're at it, maybe we can also add an extra example of how Events work?

[bjornpost]

@Jud ping for docs :-)

[Jud]

@bjornpost re: the docs, they are relatively unchanged. IMO this just makes the existing transaction code work like a casual observer would expect them to. I'll open another issue for cleaning up event docs.

Edit: btw, we've been running this code in production since ~2015. May be some corner cases we're not running into, but generally works as expected.