Don't try to index files longer than 250K chars in the background

This should fix the intermittent freezes people were seeing. We'll
process these large files when we replace nsISemanticUnitScanner.
Protect all contractions in full-text content, not just first

nsISemanticUnitScanner doesn't seem to be able to deal with single
quotes, so protect those. (There might be other characters it doesn't
handle, but this is ancient code, so it stays as is for now.)