Improve `DatabaseIndexes::bulk_index` performance #1176

Job	Run time
Build no_std (nightly)	1m 15s
Build (stable, -p nostr-sdk --features rocksdb)	17m 52s
Build (stable, -p nostr)	1m 8s
Build (1.66.0, -p nostr-sdk --features rocksdb)	2m 11s
Build FFI (stable, -p nostr-ffi)	28s
Build (stable, -p nostr --no-default-features --features alloc)	23s
Build FFI (stable, -p nostr-sdk-ffi)	3m 25s
Build WASM (stable, -p nostr)	16s
Build (stable, -p nostr --no-default-features --features "alloc all-nips")	59s
Build WASM (stable, -p nostr-sdk)	58s
Format	4s
Build (stable, -p nostr --features blocking)	1m 26s
Build WASM (stable, -p nostr-sdk --features indexeddb)	57s
Build (stable, -p nostr-database)	56s
Build (stable, -p nostr-sdk)	1m 41s
Build WASM (stable, -p nostr-js)	28s
Build (stable, -p nostr-sdk --no-default-features)	1m 0s
Build WASM (1.64.0, -p nostr)	2m 42s
Build (stable, -p nostr-sdk --features blocking)	1m 41s
Build WASM (1.64.0, -p nostr-sdk)	2m 56s
Build (stable, -p nostr-sdk --features sqlite)	2m 11s
Build WASM (1.64.0, -p nostr-sdk --features indexeddb)	3m 8s
Build (1.64.0, -p nostr)	3m 31s
Build WASM (1.64.0, -p nostr-js)	2m 54s
Build (1.64.0, -p nostr --no-default-features --features alloc)	2m 49s
Build (1.64.0, -p nostr --no-default-features --features "alloc all-nips")	3m 23s
Build (1.64.0, -p nostr --features blocking)	3m 33s
Build (1.64.0, -p nostr-database)	2m 53s
Build (1.64.0, -p nostr-sdk)	4m 4s
Build (1.64.0, -p nostr-sdk --no-default-features)	3m 20s
Build (1.64.0, -p nostr-sdk --features blocking)	3m 59s
Build (1.64.0, -p nostr-sdk --features sqlite)	4m 18s
	1h 22m 49s

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