Fix order-of-destruction error in PostingPool.
Fix order-of-destruction error in PostingPool.

Keep the MemoryPool alive while the super destructor runs so that
RawPostings allocated from it are not accessed as dangling references.

kidaa/lucy,rbevers/lucy,kidaa/lucy,pombredanne/apache-lucy,rectang/lucy,kidaa/lucy,apache/lucy,apache/lucy,nwellnhof/lucy,nwellnhof/lucy,rectang/lucy,apache/lucy,rectang/lucy,kidaa/lucy,apache/lucy,rectang/lucy,kidaa/lucy,pombredanne/apache-lucy,rectang/lucy,pombredanne/apache-lucy,rbevers/lucy,apache/lucy,kidaa/lucy,nwellnhof/lucy,nwellnhof/lucy,rbevers/lucy,rectang/lucy,nwellnhof/lucy,rectang/lucy,pombredanne/apache-lucy,rbevers/lucy,apache/lucy,pombredanne/apache-lucy,nwellnhof/lucy,pombredanne/apache-lucy,nwellnhof/lucy,kidaa/lucy,rbevers/lucy,apache/lucy,rbevers/lucy,rbevers/lucy,kidaa/lucy,pombredanne/apache-lucy