Optimize max_flushed_root update in flushing roots (#4320) #1231

Job	Run time
benchmark (solana-sdk, cargo +$rust_nightly bench -p solana-sdk --features openssl-vendored)	3m 3s
benchmark (solana-runtime, cargo +$rust_nightly bench -p solana-runtime)	1m 29s
benchmark (solana-gossip, cargo +$rust_nightly bench -p solana-gossip)	3m 38s
benchmark (solana-poh, cargo +$rust_nightly bench -p solana-poh)	1m 36s
benchmark (solana-core, cargo +$rust_nightly bench -p solana-core)	6m 10s
benchmark (sbf, make -C programs/sbf all, cargo +$rust_nightly bench --manifest-path programs/sbf...	1m 38s
benchmark (solana-accounts-db, cargo +$rust_nightly bench -p solana-accounts-db --bench accounts_...	13m 52s
	31m 26s

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