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

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).

+1 for 2.0.0 and 1.6.x from @davisp, see #510 for details.
1 file changed