Reduce default max message size

[ejona86]

Currently DEFAULT_MAX_MESSAGE_SIZE is 100 MB. That's laughably high. We should reduce it down to at least 10 MB, but 1 MB would make me feel much better.

This needs to be done cross-language.

[ejona86]

@carl-mastrangelo, would you mind picking this up and trying to coordinate a sane default maximum size across languages? You can already increase it with configuration, and we are fine with people increasing it.

[hsaliak]

/cc @ctiller @iamqizhao we need to finalize this before GA so the default behaviour is finalized..

[ejona86]

@ctiller, @a11r, and I were able to agree with going with 4 MiB for now. Future work may bring it down further, but we felt pushing much below 4 MiB required helping clients and services be in sync.