blocks: converted peak_detector2 and regenerate to v3.7.
blocks: removed peak_detector2 and regenerate blocks from gnuradio-core.
blocks: adding threshold, strech, and throttle to gr-blocks.
Merge branch 'master' into next

Conflicts:
	gr-blocks/grc/blocks_block_tree.xml
blocks: removing throttle, threshold, stretch from gnuradio-core.