Extent refactor: Introduce ecache module.

This will eventually completely wrap the eset, and handle concurrency,
allocation, and deallocation.  For now, we only pull out the mutex from the
eset.

test/unit/retained.c

@@ -142,7 +142,7 @@ TEST_BEGIN(test_retained) {
 		size_t usable = 0;
 		size_t fragmented = 0;
 		for (pszind_t pind = sz_psz2ind(HUGEPAGE); pind <
-		    arena->extent_grow_next; pind++) {
+		    arena->ecache_grow.next; pind++) {
 			size_t psz = sz_pind2sz(pind);
 			size_t psz_fragmented = psz % esz;
 			size_t psz_usable = psz - psz_fragmented;