Follow-up to r1545929:  Fix a segfault upon re-opening a connection.

* serve.c
  (serve_interruptable): Allocate the connection object in a pool
                         that has connection lifetime.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1553052 13f79535-47bb-0310-9956-ffa450edef68