use subList().clear instead of a loop of remove(0)

[XenoAmess]

use subList().clear instead of a loop of remove(0)

[coveralls]

Coverage remained the same at 90.56% when pulling 5603fb7 on XenoAmess:use_subList().clear into 715d89d on apache:master.

[XenoAmess]

Well IMO this can improve the speed as:

1. in most times the list is an ArrayList here in your codes.
2. when in original codes, each time you remove(0) it calls a System.arraycopy, means the array is copied extra times.
3. when use sublist.clear(), it actually called list.removeRange, which do only one System.arraycopy, thus far faster.

[XenoAmess]

Hi. do I need a performance benchmark for this pr too?