Fix winnt bucket_alloc to borrow memory from the transaction

pool, instead of exhausting pchild memory over a number of
connections.

PR: 11427
Submitted by: Alex Varju <alex varju.ca>
Backport: r607393

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@607394 13f79535-47bb-0310-9956-ffa450edef68