Replaces ReadAccountMapEntry in read_index_for_accessor_or_load_slow(… #11092

Job	Run time
ios (macos-11, aarch64-apple-ios)	5m 58s
ios (macos-11, x86_64-apple-ios)	6m 19s
android (ubuntu-20.04, x86_64-linux-android)	2m 48s
ios (macos-11, aarch64-apple-darwin)	6m 42s
android (ubuntu-20.04, aarch64-linux-android)	2m 58s
ios (macos-11, x86_64-apple-darwin)	6m 47s
android (ubuntu-20.04, i686-linux-android)	3m 3s
android (ubuntu-20.04, armv7-linux-androideabi)	3m 14s
error_reporting	0s
	37m 49s

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