Consider removing z_owned_reply_channel_t and z_owned_query_channel_t types and instead return both ends of the channel separately

[DenisBiryukov91]

Describe the release item

z_owned_reply_channel_t and z_owned_query_channel_t are now essentially used as containers to store both ends of the channel, which user normally immediately splits.
New API accepts a pointer to a location where an object is to be constructed. We could probably pass 2 pointers to zc_reply_fifo_new and similar functions to allow creation of both ends of the channel without introducing an extra type.

[milyin]

This issue should be taken into account when working on #325