Repeating group decoder freshly allocated on each message, violating garbage free assumptions

[writeoncereadmany]

In trying to upgrade to latest Artio, we've found that the decoders are no longer garbage free in the steady state (thanks to our allocation tests).

Specifically: when decoding messages with repeating groups, the repeating group decoder is now nulled out on reset, meaning instead of it being a lazily allocated reused field, it's now freshly allocated each decode.

This was implemented in #525: there are other potential approaches such as resetting the fields of the group decoder or giving it some sentinel value to show that it's now dirty.

[JackBeezer]

Thanks for raising this, we will take a look and update you with any progress.

[lucianoviana]

bug fix is now merged