Support to split a merged backup db file into smaller db files.
Support to split a merged backup db file into smaller db files.

If a merged backup file is too big, then it would be better to
split it into multiple smaller db files, so that each of them
can be entirely loaded into memory during online restore.

For example, if we want to split the merged backup file into
100MB backup files with the name 'squashed-xxxxx.mbb':
mbbackup-merge-incremental -o ./squashed-%.mbb -m 100 -vv ./backup*

Change-Id: Ie42a9da1a14199b089592b98c0b42a67b832d780
Reviewed-on: http://review.couchbase.org/11605
Reviewed-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>
Tested-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>

teligent-ru/ep-engine,teligent-ru/ep-engine,hisundar/ep-engine,zbase/ep-engine,abhinavdangeti/ep-engine,couchbase/ep-engine,hisundar/ep-engine,daverigby/kv_engine,abhinavdangeti/ep-engine,jimwwalker/ep-engine,sriganes/ep-engine,zbase/ep-engine,jimwwalker/ep-engine,jimwwalker/ep-engine,couchbaselabs/ep-engine,daverigby/kv_engine,couchbase/ep-engine,abhinavdangeti/ep-engine,daverigby/kv_engine,couchbaselabs/ep-engine,sriganes/ep-engine,owendCB/ep-engine,daverigby/ep-engine,teligent-ru/ep-engine,owendCB/ep-engine,couchbaselabs/ep-engine,daverigby/kv_engine,membase/ep-engine,daverigby/ep-engine,zbase/ep-engine,sriganes/ep-engine,abhinavdangeti/ep-engine,owendCB/ep-engine,couchbaselabs/ep-engine,membase/ep-engine,daverigby/ep-engine,teligent-ru/ep-engine,sriganes/ep-engine,zbase/ep-engine,couchbaselabs/ep-engine,jimwwalker/ep-engine,owendCB/ep-engine,hisundar/ep-engine,couchbase/ep-engine,abhinavdangeti/ep-engine,zbase/ep-engine,hisundar/ep-engine,membase/ep-engine,daverigby/ep-engine,couchbase/ep-engine,membase/ep-engine