Make maxRequestBytes configurable

[xichen2020]

cc @xiang90

We've been using etcd to maintain config data. On average the size of each value is a few hundred KB, but some large values can go up to a few MBs. Recently during our load testing we discovered that there is a 1.5 MB hard limit on the maximize value size for a given key. Can we make this limit configurable so we can possibly provide a different limit (slightly larger than 1.5 MB) if needed?

Our write qps is relatively low and it's okay if the requests are blocked when large keys are persisted.

[xiang90]

@xichen2020 We will add the support for this in 3.3.0

[xichen2020]

@xiang90 Ah brilliant, do you know the rough ETA for the 3.2.0 and 3.3.0 release by any chance?