Document WAL group commit and async preallocation
Document WAL group commit and async preallocation

This revision documentes WAL group commit and async preallocation in
the README file for the consensus module as to provide more context to
the source code comments in log.h and log.cc

Change-Id: Ic6319ad1301fc8a0890a79b7b72f20661c83c3a0
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1564
Tested-by: jenkins
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@cloudera.com>

helifu/kudu,EvilMcJerkface/kudu,helifu/kudu,andrwng/kudu,cloudera/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu,EvilMcJerkface/kudu,cloudera/kudu,andrwng/kudu,cloudera/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,andrwng/kudu,helifu/kudu,cloudera/kudu,andrwng/kudu,cloudera/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,cloudera/kudu,InspurUSA/kudu,helifu/kudu,andrwng/kudu,InspurUSA/kudu,cloudera/kudu,helifu/kudu,helifu/kudu,helifu/kudu,andrwng/kudu,andrwng/kudu,InspurUSA/kudu,andrwng/kudu,cloudera/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,cloudera/kudu,InspurUSA/kudu,andrwng/kudu