fuzz: better memory sizing + usage #10774

Job	Run time
ledgers / ledger-replay	1m 6s
ledgers / ledger-replay	1m 7s
ledgers / ledger-replay	1m 8s
tests / test (linux_gcc_noarch64)	1m 5s
tests / test (linux_gcc_noarch64)	1m 5s
tests / test (linux_gcc_noarch64)	1m 5s
tests / test (linux_gcc_x86_64)	1m 52s
tests / test (linux_gcc_x86_64)	8m 39s
tests / test (linux_gcc_x86_64)	8m 39s
tests / test (linux_gcc_icelake)	1m 51s
tests / test (linux_gcc_icelake)	7m 40s
tests / test (linux_gcc_icelake)	7m 40s
tests / test (linux_clang_x86_64)	2m 0s
tests / test (linux_clang_x86_64)	9m 52s
tests / test (linux_clang_x86_64)	9m 52s
tests / test (linux_clang_icelake)	1m 52s
tests / test (linux_clang_icelake)	7m 51s
tests / test (linux_clang_icelake)	7m 51s
tests / test (native)	1m 42s
tests / test (native)	8m 21s
tests / test (native)	8m 21s
tests / test (native-no-agave)	38s
tests / test (native-no-agave)	3m 12s
tests / test (native-no-agave)	3m 12s
ledgers / ledger-coverage	0s
ledgers / ledger-coverage	0s
ledgers / ledger-coverage	0s
	1h 47m 41s

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