add historic storage for keeping track of flagsets

Add an optimized historic storage to support flagsets when serving /splitChanges in the split-proxy app.

The new component is expected to return all the flags that must be returned given a change number & a list of flagsets. together with it's associations & traffic-type (this data is the minimum required to return a "fake" payload for splits that the user has to remove. This frees the proxy from keeping track of all split definitions it has seen since it started.
Based on previous stress-test results , and the reduced number of flagsets & splits, performance tends to be better with contiguos allocated memory chunks using binary search vs constructing maps to check for uniqueness & membership checks. The code gets a little uglier at some point, but reducing allocations & GC pressure proved to improve CPU usage quite a bit in the past. A benchmark was written to validate the approach. it can be run with the command cd splitio/proxy/storage/optimized && go test -bench=. -count=5 -benchmem.

Sample results here for reference:

goos: darwin
goarch: amd64
pkg: github.com/splitio/split-synchronizer/v5/splitio/proxy/storage/optimized
cpu: Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz
BenchmarkFlagSetProcessing/sorted-slice-16                  6470            194996 ns/op          119632 B/op        190 allocs/op
BenchmarkFlagSetProcessing/sorted-slice-16                  7689            184180 ns/op          120015 B/op        191 allocs/op
BenchmarkFlagSetProcessing/sorted-slice-16                  7058            179562 ns/op          120483 B/op        192 allocs/op
BenchmarkFlagSetProcessing/sorted-slice-16                  7476            183769 ns/op          119587 B/op        190 allocs/op
BenchmarkFlagSetProcessing/sorted-slice-16                  7656            178133 ns/op          119867 B/op        191 allocs/op
BenchmarkFlagSetProcessing/maps-16                          1940            605115 ns/op         1535541 B/op       2380 allocs/op
BenchmarkFlagSetProcessing/maps-16                          1942            615861 ns/op         1534754 B/op       2379 allocs/op
BenchmarkFlagSetProcessing/maps-16                          1915            615651 ns/op         1535475 B/op       2380 allocs/op
BenchmarkFlagSetProcessing/maps-16                          2038            635693 ns/op         1534103 B/op       2378 allocs/op
BenchmarkFlagSetProcessing/maps-16                          1953            606300 ns/op         1535843 B/op       2380 allocs/op
PASS

splitio / split-synchronizer

add historic storage for keeping track of flagsets #242