Expose MessageLimit as a configuration

[zhzy0077]

Problem:
When sending long messages, the server returns

{
    "code": 41303,
    "http": 413,
    "error": "JSON body too large"
}

Changes:
Expose the MessageLimit as a configuration key so users can customize this value.

[binwiederhier]

I am willing to accept this as a config option if you document it properly in the config.md like the other options with a big fat warning around the fact that if a higher message limit is used with FCM or APNS, things will likely not work at all.

[zhzy0077]

@binwiederhier Any other concerns of this PR?