Clear the localThread when the index is refreshed

Clean up the local thread after each sites when doing a complete
reindexation or a complete refresh.
This cleanup should be in the larger tasks, not the smaller ones
Return null streams when file is too big

When a file is too heavy to be indexed, return a null stream, the
system will have to check that the stream is never null
Fix a confusing log message about file max size

The log shouldn't say "expected size", it's just the maximum supported
Merge branch 'master' into search-1.3.x