writeall and writemultiple are not conforming to the TD spec

[danielpeintner]

... writeall and writemultiple are not conforming to the TD spec. They do not seem to accept a single request to write into all the properties but as done in consumed-thing.ts, all of them has to be written separately.

Originally posted by @egekorkan in #949 (comment)

[relu91]

It is the same for readallproperties and readmultipleproperties right? The funny thing is that the Modbus binding has a feature for detecting a series of open requests and it can actually merge them into one single request. The result is that for Modbus even if you have multiple calls to "read" or "write" they are packed into one single network request. However, this is a very protocol-specific behavior and probably it won't work with HTTP... for HTTP the problem is that in different application protocols, we'll have different ways to represent this "aggregated" operation. I think it is something it should be addressed in the TD spec.