resmoke.py's logkeeper client should respect size limit for POST requests by splitting them up

With the changes from 4f0b759, Logkeeper will reject POST requests larger than 32MB. In order to avoid hardcoding this constant in resmoke.py's logkeeper client (e.g. should the value need to be lowered further on the Logkeeper application), the changes from EVG-1365 made it so that rejected POST requests will return a "Request entity too large" HTTP 413 error status with a "max_size_mb" field containing the size limit for POST requests in the error response.

buildscripts/resmokelib/logging/buildlogger.py should be updated so that upon receiving this HTTP error response, it retries the request with the JSON-encoded content split up into "max_size_mb" chunks.