core: Optimize string concatenation in expression
parser when evaluating a string expression.

We have a relatively small recursion limit of
about 10 operations. This is a compilation
limit (a define). It can be hit if many expr
vars or function calls are concatenated in an
expression that is evaluated as a string not as
a boolean.

The new optimization uses iteration instead of
recursion and is very similar to the
existing one, which optimizes consecutive
concatenations in node2 of the tree. The new
one optimizes consecutive concatenations in
node 1.

Submitted by: rjung
Reviewed by: rjung, ylavic, covener
Backported by: rjung


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1662075 13f79535-47bb-0310-9956-ffa450edef68