DDS15 — Allow for more optimized list returned by get_datareaders()

Found by: Fernando Crespo
Severity:

Problem:
OMG DDS spec (section 2.2.2.5.2.10) states that the sequence returned by
get_datareaders() will contain list containing each DataReader one or more times.
For example, if multiple consecutive samples in a group belong to the same DataReader
the DataReader is repeated in the list returned by get_datareaders().
Having to process each element, even when they belong to the same DataReader is
less performant.

Proposed Resolution:
Modify the specification to return one DataReader element, instead of a list where
a DataReader is repeated multiple times, when multiple subsequent samples belong to
the same DataReader. This allows for more optimized processing where the user calls
read/take until the return code is NO_DATA.