SDRplay rewrite w/RSP2

I picked up a SDRplay RSP2 recently, and the gr-osmosdr support needed some work. Here's a rewrite that works with RSP2 and has other improvements. When the RSP1A comes in, I'll finish support for that. No plans to pick up a RSP1, but someone can see if this works if interested. This was tested against the Nov 2017 binary blob.

https://github.com/willcode/gr-osmosdr/tree/sdrplay2

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Complete rewrite of sdrplay support

- Supports RSP2 - Will support RSP1A when hardware arrives - RSP1 is untested - Uses streaming callbacks instead of polling - Tuning, gain control imporoved, AGC supported - Better performance (one less layer of buffers)