This is much more fiddly than you'd expect it to be because of the way
position_offset_gap is applied in StringFieldMapper. Instead of setting
the default to 100 its simpler to make sure that all the analyzers default
to 100 and that StringFieldMapper doesn't override the default unless the
user specifies something different. Unless the index was created before
2.1, in which case the old default of 0 has to take.
Also postition_offset_gaps less than 0 aren't allowed at all.
New tests test that:
1. the new default doesn't match phrases across values with reasonably low
slop (5)
2. the new default doest match phrases across values with reasonably high
slop (50)
3. you can override the value and phrases work as you'd expect
4. if you leave the value undefined in the mapping and define it on a
custom analyzer the the value from the custom analyzer shines through
Closes#7268
Nik Everett
Adrien Grand