when debug iterators are enabled enable them in boost-multi-index #2908

Job	Run time
Build (crypto=built-in)	5m 54s
Simulations	9m 30s
Build (release)	7m 29s
Tests (crypto=built-in)	12m 25s
Tests (webtorrent=on deprecated-functions=off)	19m 14s
Build iOS	3m 5s
	57m 37s

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