optionally slowdown the collapse process
test for collapse_slowdown option
Turn `collapse_slowdown` into a float option

Also add some documentation in the sample configuration file.
Merge branch 'origin-1.6' into origin-slowdown-collapse-1.6

Conflicts:
	extension/test/server/system_tests_common.py
fix merge error
Merge remote-tracking branch 'refs/remotes/origin/slowdown-collapse-1.6' into slowdown-collapse-1.7

Conflicts:
	cfg/arakoon.ini
	extension/server/ArakoonManagement.py
	extension/test/server/system_tests_common.py
	src/node/collapser.ml
	src/node/collapser_test.ml
	src/node/node_cfg.ml
	src/node/node_main.ml
	src/node/sync_backend.ml
	src/system/startup.ml
Merge branch 'incubaid-1.7' into slowdown-collapse-1.7

Conflicts:
	src/node/sync_backend.ml
lift slowdown into wrapper function
Merge pull request #308 from Incubaid/slowdown-collapse-1.7

Slowdown collapse 1.7