This is all @dirkx , whose PR unfortunately got borked when we were
trying to update it to the new format. As @dirkx said:
When sending POST responses of well over a K - _write() may not sent it
all. Make sure we do -- but cap the individual writes - as somehow large
2-3k blobs seem to cause instability (on my 12F units).
Supercedes #2528
8f6e0dd339