Define execute operation on Request; remove read/write operations from Reader/Writer

[Andrey Skorikov]

Hello,

currently, PlcReadRequests and PlcWriteRequests are executed by invoking 
the corresponding read/write operation on the PlcReader/PlcWriter 
objects. Hence the typical workflow for reading a value contains the 
following steps:

1. Obtain a PlcReader instance from a PlcConnection
2. Obtain a Builder by invoking readRequestBuilder() on PlcReader
3. Build a request using the Builder
4. Execute the request by invoking read() on the PlcReader, passing the 
constructed request as argument

PlcReader reader = ... // obtain reader
PlcReadRequest request = reader.readRequestBuilder().addItem(...).build();
Future<PlcReadResponse> response = reader.read(request);

Since we only can build a request throgh a PlcReader instance, invoking 
the read operation on the PlcReader is redundant. Therefore I suggest 
removing the read/write operation from the PlcReader/PlcWriter and 
defining a execute() operation on PlcRequest instead. The workflow would 
look like this then:

PlcReader reader = ... // obtain reader
PlcReadRequest request = reader.readRequestBuilder().addItem(...).build();
Future<PlcReadResponse> response = request.execute();

Please note that there is no more need to "keep" a reference to 
PlcReader in this context, since it is implicitly associated with the 
request by calling reader.readRequestBuilder().build().

Best regards,
Andrey