First cut for sampling at 2.4MHz + phase detection.
Fix preamble quiet-bits check.
Oversampling, round two.

This now seems to be at the point where it will decode more messages
than when using 2MHz with --phase-enhance.
Tweak correlator weighings to be right, and scale up some more to avoid losing precision.

Further tweaking to which phase offsets we look for.
Fix mlat inter-block timestamp accounting in oversampling mode.
Add calculated phase into the per-message timestamp (it is already exactly a 12MHz offset)
Log message time and symbol phase when dumping message info.
Add SNR calculation in oversample mode.
Performance tweaking for AGC.

Apparently enabling AGC produces samples with quite different characteristics,
and ends up eating a lot more CPU as the previous heuristics would generate a
lot of false positives. Tweaking the parameters and a bit of optimization
seems to bring this back down to usable levels without losing many potential
messages.
Measure CPU used by the sample processing thread.
Move CPU instrumentation up into the main loop.
More tweaking to try to get the signal / CPU tradeoff right.
Try all phases if --oversample --phase-enhance is on.

If we demodulate a message in 2.4MHz mode and it has a bad, uncorrectable CRC,
and --phase-enhance is on, then retry with the other possible phases until
we get a good CRC or run out of phases to try.

This is very expensive in AGC mode (lots of candidates that are not real
messages) but relatively cheap otherwise. It yields another 10% messages.

Also factor out some common stats code to avoid lots more copy/paste.
Use uint64_t in load stats to avoid overflow with large intervals.
Resync with master.
Merge branch 'master' into oversampling
Merge branch 'fix-net-list-corruption' into oversampling
Merge branch 'net-cleanups' into oversampling

Conflicts:
	dump1090.c
	dump1090.h
Merge branch 'master' into oversampling
Experimental json changes
Merge branch 'write-json-files' into oversampling

Conflicts:
	dump1090.c
	dump1090.h