add memory profiling utility functions
scan optimisation: cache blacklists
changes config: ref #99: oversize release processing changes

new config options:
  - max_process_size: upper limit on the size of releases to process, which triggers:
  - max_process_anyway: whether to swap to slow-process of large releases or to just delete them outright and not index them
nzb creation optimisation

build it using stringio instead of stringcats, which are pretty slow