Use user's memory segments for doc's meta & body
Use user's memory segments for doc's meta & body

- If user manually assign memory segments for document's meta or
body section, do not allocate new memory but use the given memory
segments.

Change-Id: I0172cc188a917f8e2b3fa7bc0aded4960bae0320

jinxuan/forestdb,couchbase/forestdb,sushmaad/forestdb,yyangpan/forestdb,jinxuan/forestdb,wurikiji/forestdb,wurikiji/forestdb,sushmaad/forestdb,zhaog/forestdb,couchbase/forestdb,yyangpan/forestdb,nvoron23/forestdb,zhaog/forestdb,seem-sky/forestdb,hisundar/forestdb,seem-sky/forestdb,joliss/forestdb,nvoron23/forestdb,joliss/forestdb,hisundar/forestdb