etcd tester runs etcd runner as a separate binary.
it signals sigstop to the runner when tester wants to stop stressing.
it signals sigcont to the runner when tester wants to start stressing.
when tester needs to clean up, it signals sigint to runner.
FIXES#7026
functional tester sometime experiences timeout during compaction phase. I changed the timeout calculation base on number of entries created and deleted.
FIX#6805
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
