[drsyms] Manage module cache to prevent run away memory usage

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on May 06, 2011 11:55:14_

filing an issue from the drsyms_windows.c implementation:
- TODO PR 463897: be more robust about handling failures packing in
- loaded modules. E.g., today we will probably fail if passed two
- .exe's (non-relocatable). See further comments in load_module()
- below.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=449_