save 16 bytes per WAL item by list_elem_txn
save 16 bytes per WAL item by list_elem_txn

wal item can't be in transaction list and flush tree
at same time. Putting it in the union to save 16 bytes per
WAL item which translates to a big gain during compaction &
with large wal sizes

Change-Id: Ifb419507e6d8b0651c82a1f85b1fc371a18d73f8

wurikiji/forestdb,wurikiji/forestdb,couchbase/forestdb,hisundar/forestdb,couchbase/forestdb,hisundar/forestdb