Refactor ignore handling (...)

This uses persistent Matcher objects that can reload their content and
provide a hash string that can be used to check if it's changed. The
cache is local to each Matcher object instead of kept globally.
Move ignore handling from index recv to puller (fixes #1133)

With this change we accept updates for ignored files from other devices,
and check the ignore patterns at pull time. When we detect that the
ignore patterns have changed we do a full check of files that we might
now need to pull.
Merge pull request #1140 from syncthing/fix-1133

Refactor ignore handling to fix #1133