Hibernate couch_stream after each write

In COUCHDB-1946 Adam Kocoloski investigated a memory explosion resulting
from replication of databases with large attachments (npm fullfat). He
was able to stabilize memory usage to a much lower level by hibernating
couch_stream after each write. While this increases CPU utilization when
writing attachments, it should help reduce memory utilization.

This patch is the single change that affected a ~70% reduction in
memory.

No alteration to the spawn of couch_stream to change the fullsweep_after
setting has been made, in part because this can be adjusted at the erl
command line if desired (-erl ERL_FULLSWEEP_AFTER 0).

src/couch/src/couch_stream.erl

@@ -259,7 +259,7 @@ handle_call({write, Bin}, _From, Stream) ->
  buffer_len=0,
  md5=Md5_2,
  identity_md5=IdenMd5_2,
- identity_len=IdenLen + BinSize}};
+ identity_len=IdenLen + BinSize}, hibernate};
  true ->
  {reply, ok, Stream#stream{
  buffer_list=[Bin|Buffer],