[SPARK-8077] [SQL] Optimization for TreeNodes with large numbers of children

For example large IN clauses

Large IN clauses are parsed very slowly. For example SQL below (10K items in IN) takes 45-50s.

s"""SELECT * FROM Person WHERE ForeName IN ('${(1 to 10000).map("n" + _).mkString("','")}')"""

This is principally due to TreeNode which repeatedly call contains on children, where children in this case is a List that is 10K long. In effect parsing for large IN clauses is O(N squared).
A lazily initialised Set based on children for contains reduces parse time to around 2.5s

Author: Michael Davies <Michael.BellDavies@gmail.com>

Closes #6673 from MickDavies/SPARK-8077 and squashes the following commits:

38cd425 [Michael Davies] SPARK-8077: Optimization for  TreeNodes with large numbers of children
d80103b [Michael Davies] SPARK-8077: Optimization for  TreeNodes with large numbers of children
e6be8be [Michael Davies] SPARK-8077: Optimization for  TreeNodes with large numbers of children