MB-16263: allow invalidated index blocks to be serialized
MB-16263: allow invalidated index blocks to be serialized

Considering dirty immutable index blocks as doc blocks allows
for better serialization of I/O during the background flush
This also reduces the number of memory management calls issued.
In performance runs this results in small (5%) improvements but
greater gains may be seen with O_DIRECT.

Change-Id: I3d42d58a8bf6b2cd5f840a9048741a862a0eb142

hisundar/forestdb,wurikiji/forestdb,wurikiji/forestdb,hisundar/forestdb,couchbase/forestdb,couchbase/forestdb