Fix broken mlock call

Should be a MlockRaw, not an Mlock. Got broken in the transition to Base::Mlock.
Add missing exception
posix_malign returns non-zero on error

The return code is likely positive to indicate the error code.
Lower the ram requirements of index_manager.test
Merge pull request #45 from cmaloney/mlock_fixes

Misc Fixes around the Mlock pull request
Merge pull request #46 from cmaloney/reduce_test_memory

Reduce memory usage of update_walk_file and present_walk_file unit tests
Further lower the memory requirements of the index manager test

They could be lowered further than they were with the last change.