fix: benchmark with multiple producers

[fraidev]

This PR fixes benchmark with many producers and the output interval for latencies:

▷ RELEASE=true make build-cli && flvdr bench producer --num-records 300000
# Fluvio Benchmark Results
created topic benchmark-mrnxowicwc
starting up producer 0
Benchmark started
producer send batch
producer is sending batch
27412 records sent, 41787 records/sec: (213.5 MB/sec), 0.72ms avg latency, 2.59ms max latency
42644 records sent, 42304 records/sec: (216.2 MB/sec), 0.70ms avg latency, 7.33ms max latency
42840 records sent, 42506 records/sec: (217.2 MB/sec), 0.70ms avg latency, 1.93ms max latency
42420 records sent, 42482 records/sec: (217.1 MB/sec), 0.71ms avg latency, 6.18ms max latency
42168 records sent, 42415 records/sec: (216.8 MB/sec), 0.72ms avg latency, 1.90ms max latency
41356 records sent, 42228 records/sec: (215.8 MB/sec), 0.73ms avg latency, 2.42ms max latency
41944 records sent, 42185 records/sec: (215.6 MB/sec), 0.72ms avg latency, 1.97ms max latency

0.71ms avg latency, 7.33ms max latency, 0.69ms p0.50, 0.80ms p0.95, 1.79ms p0.99
300000 total records sent, 42188 records/sec: (215.6 MB/sec) 
Benchmark completed

[sehz]

double spawn loop is inefficient. it may contribute to some latency. not show stopper for this PR, but should flatten this. it's probably better perform accumulator with atomic counter callback then perform analytics in the stat collector