opensearchserver_data set to [user.home]/opensearchserver_data if not
set
Implements sorting on joined index
Partial implementation of join sort
Add support of empty values in sorting #403. Empty values can now be
forced in first or last position.
New sorting UI (#403)
Add support of join sort in search API (v1) #403

Syntax: N-fieldname_$null$
It means: 
N: The number of the join. 0 means no joins.
- or +: The direction of the sort
fieldname: the name of the field
_$null$: Null values comes first
Merge pull request #404 from emmanuel-keller/1.5

Sorting improvement, home user used by default