fuzz: better memory sizing + usage #10775

Job	Run time
ledgers / ledger-replay	1m 8s
ledgers / ledger-replay	1m 7s
tests / test (linux_gcc_noarch64)	1m 14s
tests / test (linux_gcc_noarch64)	1m 14s
tests / test (linux_gcc_x86_64)	5m 25s
tests / test (linux_gcc_x86_64)	12m 7s
tests / test (linux_gcc_icelake)	5m 20s
tests / test (linux_gcc_icelake)	7m 33s
tests / test (linux_clang_x86_64)	5m 26s
tests / test (linux_clang_x86_64)	10m 58s
tests / test (linux_clang_icelake)	5m 14s
tests / test (linux_clang_icelake)	7m 49s
tests / test (native)	5m 26s
tests / test (native)	7m 57s
tests / test (native-no-agave)	3m 7s
tests / test (native-no-agave)	3m 7s
ledgers / ledger-coverage	0s
ledgers / ledger-coverage	0s
	1h 24m 12s

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