tszlong: make encoding more compact by conservatively resetting leading/trailing bitcount

[shanson7]

While profiling memory usage for Metrictank we found that ~15% of memory usage came from the raw bstream. This wasn't surprisingly high, but we decided to look for possible optimizations.

We found that 67% of those allocations came from this one line. This line is for writing the significant bits when the value isn't exactly the same as the previous, but falls within the leading/trailing zeroes that previously had been written. However, since the leading/trailing is never reset, a single errant value can cause a lot of significant bits to be written each time. This is especially true for floating point values. For example (playground):

math.Float64bits(126.45) ^ math.Float64bits(127.45) only has a difference of 1 bit (17 leading zeroes/46 trailing)
math.Float64bits(127.45) ^ math.Float64bits(128.45) has only 10 leading zeroes and 0 trailing.

So in a lot of cases we could end up writing many bits of extra sig figs (lots of leading/trailing zeroes) when it could be better to just re-encode the leading/trailing zeroes and write fewer sig figs for some amount of time.

I added some more scenarios to the benchmarks to test various data patterns, 120 datapoints each (chosen as that is the recommended chunk size):

Pattern	Old Bytes	New Bytes	% of Old
Mono+reset	993	384	38.67%
Mono	843	162	19.2%
SawTooth	1008	1006	99.8%
SawTooth+Flat	1008	1004	99.6%
Steps	752	703	93.48%
Real World CPU	777	671	86.36%

As seen, this approach performs no worse than the existing in terms of chunk size, however in some cases it is significantly better. I have yet to plug this into a running MT instance and see if the result is true across a large range of real-world data.

[shanson7]

We saw an average 4-byte per chunk reduction across all chunks. This represents only ~3% improvement, but better than nothing.

[Dieterbe]

ready to merge. see comment, waiting for your final go ahead.

[shanson7]

We have been running this code in production and saw a small average benefit. Many series see no change (example series that just publish integers). I'm good with merging this change as-is.