Changed healthy status to be sent when task killed after health check.

Currently after the health check receives configured amount of task
fails it will initiate a kill task. However currently the task kill
status update doesn't set the healthy field so it's unclear was it
killed because health check or other reasons. This patch sets the health
check field when the task is killed because of failing health checks.

Review: https://reviews.apache.org/r/23520
Fixed variable naming in a test.
Renamed slaves/frameworks activated/deactivated

The existing terminology is confusing both for "slaves.deactivated" and
"frameworks.activated". Currently a deactivated slave actually
represents a removed/shutdown slave and "frameworks.activated" map holds
both activated and deactivated frameworks.
In order to make things look clear, rename the following:
* master.slaves.deactivated -> master.slaves.removed
* master.slaves.activated -> master.slaves.registered
* master.frameworks.activated -> master.frameworks.registered
* allocator.slaveDisconnect -> allocator.slaveDeactivate
* allocator.slaveReconnected -> allocator.slaveReactivated

Review: https://reviews.apache.org/r/23147
Re-enabled health check tests

Currently the health check tests are disabled as it was flaky. Part of the reason is that all assertions was EXPECTs instead of ASSERTs so it will continue to execute.
Another issue was it's currently discovering the path to launch health check executable from the command executor main argv[0] path.
However, the correct path is to launch the generated script that does library path resolution, which is the parent of .libs.

Review: https://reviews.apache.org/r/23443
Fixed a deadlock bug in Mutex.

Review: https://reviews.apache.org/r/23592
Disabled flaky low level scheduler tests.