Add dynamic thresholds to the GC

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[nascheme]

Below is a summary of GC behavior while testing a web application which creates some cyclic trash. It is running with Python 3.11.1 and with this dynamic threshold PR logic. The dynamic version seems effective in reducing the time spent in GC and doesn't increase thresholds too much so there is a lot of cyclic trash wasting memory. IMHO, this idea could be worth pursuing. The high thresholds for gen1 and gen2 are too high, IMHO, since they are based on number of gen0 and gen1 collections, not #-of-objects. The magic constants (e.g. 10 and 5 in exponent = -((ratio - target)*10.0-5) probably need some tuning. The ratio of collected objects in gen0 jumps around on each collection. See some log output generated from gc_adapt_threshold below. The long term average ratio collected in gen0 is not approaching the target 0.2 and so maybe the adapt function needs some tweaking.

branch	gen	num collections	mean collected cycles	mean time [ns]	mean total objects	total time [s]	time per total [ns/obj]	time per collected [ns/obj]	ratio collected
main	0	16272	34	56,313	739	0.92	76	1653	5%
main	1	1478	995	726,633	4,827	1.07	151	730	21%
main	2	106	21,118	80,731,000	210,969	8.56	383	3823	10%
dynamic	0	2718	343	357,774	3,095	0.97	116	1042	11%
dynamic	1	163	15,153	7,458,847	24,406	1.22	306	492	62%
dynamic	2	10	91,193	147,900,000	282,123	1.48	524	1622	32%

gc threshold: gen 1 ratio 0.83 old 10 new 10
gc threshold: gen 0 ratio 0.05 old 2555 new 2627
gc threshold: gen 0 ratio 0.06 old 2627 new 2692
gc threshold: gen 0 ratio 0.12 old 2692 new 2726
gc threshold: gen 0 ratio 0.15 old 2726 new 2752
gc threshold: gen 0 ratio 0.14 old 2752 new 2778
gc threshold: gen 0 ratio 0.17 old 2778 new 2798
gc threshold: gen 0 ratio 0.76 old 2798 new 1446
gc threshold: gen 0 ratio 0.54 old 1446 new 1320
gc threshold: gen 0 ratio 0.41 old 1320 new 1286
gc threshold: gen 0 ratio 0.21 old 1286 new 1281
gc threshold: gen 0 ratio 0.49 old 1281 new 1218
gc threshold: gen 1 ratio 0.41 old 10 new 10
gc threshold: gen 0 ratio 0.18 old 1218 new 1247
gc threshold: gen 0 ratio 0.02 old 1247 new 1397
gc threshold: gen 0 ratio 0.13 old 1397 new 1446
gc threshold: gen 0 ratio 0.14 old 1446 new 1489
gc threshold: gen 0 ratio 0.14 old 1489 new 1531
gc threshold: gen 0 ratio 0.13 old 1531 new 1576
gc threshold: gen 0 ratio 0.14 old 1576 new 1618
gc threshold: gen 0 ratio 0.14 old 1618 new 1660
gc threshold: gen 0 ratio 0.14 old 1660 new 1700
gc threshold: gen 0 ratio 0.15 old 1700 new 1734
gc threshold: gen 0 ratio 0.70 old 1734 new 1206
gc threshold: gen 1 ratio 0.15 old 10 new 16
gc threshold: gen 0 ratio 0.46 old 1206 new 1164
gc threshold: gen 0 ratio 0.32 old 1164 new 1153
gc threshold: gen 0 ratio 0.54 old 1153 new 1075
gc threshold: gen 0 ratio 0.51 old 1075 new 1028
gc threshold: gen 0 ratio 0.56 old 1028 new 961
gc threshold: gen 0 ratio 0.58 old 961 new 900
gc threshold: gen 0 ratio 0.21 old 900 new 898
gc threshold: gen 0 ratio 0.07 old 898 new 999
gc threshold: gen 0 ratio 0.20 old 999 new 1026
gc threshold: gen 0 ratio 0.34 old 1026 new 1017
gc threshold: gen 0 ratio 0.35 old 1017 new 1007
gc threshold: gen 0 ratio 0.39 old 1007 new 993
gc threshold: gen 0 ratio 0.50 old 993 new 959
gc threshold: gen 0 ratio 0.45 old 959 new 939
gc threshold: gen 0 ratio 0.02 old 939 new 1103
gc threshold: gen 0 ratio 0.01 old 1103 new 1263
gc threshold: gen 0 ratio 0.12 old 1263 new 1318
gc threshold: gen 1 ratio 0.79 old 16 new 14
gc threshold: gen 0 ratio 0.04 old 1318 new 1437
gc threshold: gen 0 ratio 0.08 old 1437 new 1516
gc threshold: gen 0 ratio 0.03 old 1516 new 1645
gc threshold: gen 0 ratio 0.11 old 1645 new 1698