Disable coalescing of cached extents.

Extent splitting and coalescing is a major component of large allocation
overhead, and disabling coalescing of cached extents provides a simple
and effective hysteresis mechanism.  Once two-phase purging is
implemented, it will probably make sense to leave coalescing disabled
for the first phase, but coalesce during the second phase.

include/jemalloc/internal/extent_structs.h

@@ -115,6 +115,9 @@ struct extents_s {
 
 	/* All stored extents must be in the same state. */
 	extent_state_t		state;
+
+	/* If true, try to coalesce during extent deallocation. */
+	bool			try_coalesce;
 };
 
 #endif /* JEMALLOC_INTERNAL_EXTENT_STRUCTS_H */