Inefficient shrinking in optimization mode

[rappie]

The issue is perfectly summarized in this comment: #75 (comment)

I'm currently using a shrinkLimit of 100k (which is still not enough to fully shrink). This means shrinking takes a long time, and as medusa will run through all new discovered sequences, this takes around 30 hours (16 workers, fast cpu) to get through the existing corpus before it starts shrinking the most optimized sequences.

A simple solution that would work fine for me is to do the same as Echidna: start shrinking after testLimit is reached.

Example log for reference (using 1 worker)

⇾ fuzz: elapsed: 0s, calls: 0 (0/sec), seq/s: 0, coverage: 93015, corpus: 169, failures: 0/0, gas/s: 0
⇾ [Worker 0] Shrinking call sequence with 1 call(s)
⇾ [Worker 0] Shrinking call sequence with 3 call(s)
⇾ [Worker 0] Shrinking call sequence with 3 call(s)
⇾ fuzz: elapsed: 3s, calls: 31 (10/sec), seq/s: 6, coverage: 93015, corpus: 169, failures: 0/19, gas/s: 5834726
⇾ fuzz: elapsed: 6s, calls: 31 (0/sec), seq/s: 0, coverage: 93015, corpus: 169, failures: 0/19, gas/s: 0
⇾ [Worker 0] Shrinking call sequence with 4 call(s)
⇾ fuzz: elapsed: 9s, calls: 43 (3/sec), seq/s: 0, coverage: 93015, corpus: 169, failures: 0/22, gas/s: 4307595
⇾ fuzz: elapsed: 12s, calls: 43 (0/sec), seq/s: 0, coverage: 93015, corpus: 169, failures: 0/22, gas/s: 0
⇾ [Worker 0] Shrinking call sequence with 5 call(s)
⇾ fuzz: elapsed: 15s, calls: 52 (2/sec), seq/s: 0, coverage: 93015, corpus: 169, failures: 0/24, gas/s: 3452692
⇾ fuzz: elapsed: 18s, calls: 52 (0/sec), seq/s: 0, coverage: 93015, corpus: 169, failures: 0/24, gas/s: 0
⇾ [Worker 0] Shrinking call sequence with 5 call(s)
⇾ fuzz: elapsed: 21s, calls: 63 (3/sec), seq/s: 0, coverage: 93015, corpus: 169, failures: 0/26, gas/s: 3635980
⇾ fuzz: elapsed: 24s, calls: 63 (0/sec), seq/s: 0, coverage: 93015, corpus: 169, failures: 0/26, gas/s: 0
⇾ [Worker 0] Shrinking call sequence with 5 call(s)
⇾ fuzz: elapsed: 27s, calls: 135 (23/sec), seq/s: 3, coverage: 93015, corpus: 169, failures: 0/38, gas/s: 23818187
⇾ fuzz: elapsed: 30s, calls: 135 (0/sec), seq/s: 0, coverage: 93015, corpus: 169, failures: 0/38, gas/s: 0
⇾ [Worker 0] Shrinking call sequence with 6 call(s)
⇾ fuzz: elapsed: 33s, calls: 154 (6/sec), seq/s: 0, coverage: 93015, corpus: 169, failures: 0/41, gas/s: 6674950