[Feature request] Disconnect streamer from RFNoC graph

[waterfood128]

Issue Description

Right now there is not a proper way to disconnect a streamer once it is connected to a RFNoC block. There are two sub-issues

1. rfnoc_graph::disconnect() takes std::string streamer_id argument, however there is no public API to get streamer_id. Users need to guess the id (by looking at source code)
2. uhd::transport::tx_streamer_zero_copy never clears _xports so users cannot call connect_channel() twice. Users need to create a new streamer object to workaround this.

Setup Details

I use replay block to buffer both tx/rx samples for all channels, which use all replay block's ports. When I want to write/read samples from host, I need to disconnect one port, connect it to the streamer, and disconnect when I'm done. Current public API does not support this use case.

Expected Behavior

Have an API rfnoc_graph::disconnect(uhd::tx_streamer& streamer, size_t port)

[mbr0wn]

As mentioned on the mailing list, there already exists a "correct" way to disconnect the streamer, which is to destroy the streamer object.

[waterfood128]

I wonder if it's ok to document the correct way in API doc, specifically, on rfnoc_graph::[create_tx_streamer() and rfnoc_graph::[create_rx_streamer(). Also, deprecate rfnoc_graph::disconnect(streamer_id) to avoid confusion.