The checkers and stressers should be composable without special cases; this
patch tries to address that while refactoring out some old cruft.
Namely,
* Single stresser/checker for a tester; built from composition
* Composite stresser via comma-separated list of stressers
* Split stressers into separate files
* Removed v2 only flags and special cases
* Rate limiter shared among key stresser and leases stresser
* Composite checker is now concurrent
* Stresser can return a Checker to check its invariants
* Each lease checker only operates on a single lease stresser
lease stresser now generates short lived leases that will expire before invariant checking.
this addition verifies that the expired leases are indeed being deleted on the sever side.
increasing lease TTL ensure that lease doesn't expire during hashes stabilization period.
I observed that it can take a long time for etcd cluster to become stable.
