Merged from trunk 10545:10553

   Merged from trunk 10545:10553

   | 10546   | Don't unset PORTAGE_BINPKG_FILE because it's readonly.       |
   | zmedico |                                                              |

   | 10547   | Add support for a PORTAGE_RO_DISTDIRS variable. When a given |
   | zmedico | file does not exist in DISTDIR, search for the file in this  |
   |         | list of directories. Search order is from left to right.     |
   |         | Note that the current implementation works by creating a     |
   |         | symlink inside DISTDIR, but that may change in the future.   |

   | 10548   | Fix vardbapi.flush_cache() so that it only updates the cache |
   | zmedico | when the number of uncached packages reaches a certain       |
   |         | threshold (currently 5). The cache file can be several       |
   |         | megabytes in size, so updating it for every vdb change is    |
   |         | wasteful.                                                    |

   | 10549   | Add modification threshold for BlockerCache.flush(), to      |
   | zmedico | avoid wasteful IO.                                           |

   | 10550   | Use float mtime instead of int for finer granularity in      |
   | zmedico | vardbapi.match() cache staleness checks.                     |

   | 10551   | When pruning the digests to optimize parallel-fetch, use a   |
   | zmedico | separate dict so that the original digests are still         |
   |         | available if a file needs to be downloaded.                  |

   | 10552   | Fix typo.                                                    |
   | zmedico |                                                              |

   | 10553   | Tweak the installed masked code from bug #223447 so that it  |
   | zmedico | less noisy by suppressing masking messages for packages that |
   |         | haven't been pulled into the current dependency graph.       |

svn path=/main/branches/prefix/; revision=10556