Service ID zones leak malloc memory

[rmacnak-google]

Id zone allocation is implemented by appending to a list and deletion is implemented by replacing the element with null. Repeatedly allocating and deleted id zones thus consumes memory proportional to the number of id zones ever created rather then number currently alive. Repeatedly allocating and deleting id zones is an expected usage pattern for service clients performing high level operations that require multiple service RPCs.

@bkonyi @jyameo