Update to airlift 201
Update to airlift 201

This version contains some fixes to t-digest and is a pre-requisite
to being able to add the t-digest type.

11xor6/presto,electrum/presto,Praveen2112/presto,smartnews/presto,erichwang/presto,11xor6/presto,Praveen2112/presto,losipiuk/presto,erichwang/presto,ebyhr/presto,ebyhr/presto,11xor6/presto,11xor6/presto,Praveen2112/presto,electrum/presto,smartnews/presto,11xor6/presto,erichwang/presto,erichwang/presto,electrum/presto,smartnews/presto,smartnews/presto,smartnews/presto,dain/presto,electrum/presto,dain/presto,ebyhr/presto,Praveen2112/presto,electrum/presto,losipiuk/presto,dain/presto,ebyhr/presto,dain/presto,erichwang/presto,losipiuk/presto,dain/presto,losipiuk/presto,losipiuk/presto,ebyhr/presto,Praveen2112/presto