metadump: ensure buffer is flushed before finish
metadump: ensure buffer is flushed before finish

if more than one write/poll() worth of data is in the bipbuf but the crawl is
complete, the client might get released back with an incomplete write.

elfchief/memcached,bpzhang/memcached,bpzhang/memcached,dormando/memcached,elfchief/memcached,gencer/memcached,memcached/memcached,elfchief/memcached,bpzhang/memcached,dormando/memcached,dormando/memcached,elfchief/memcached,gencer/memcached,gencer/memcached,guikcd/memcached,memcached/memcached,guikcd/memcached,guikcd/memcached,bpzhang/memcached,elfchief/memcached,memcached/memcached,dormando/memcached,gencer/memcached,dormando/memcached,memcached/memcached,gencer/memcached,memcached/memcached,guikcd/memcached,guikcd/memcached