Fix a bug in appending a large document
Fix a bug in appending a large document

- When a document that is longer than at least 3 blocks (i.e., 12 KB)
is appending, block markers for the middle blocks are overwritten with
an arbitrary data due to use of filemgr_write.

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