`InstanceSpec`'s `customProperties` will be fed to `Properties` in
`QuorumConfigBuilder::buildConfigProperties`, so we have to make sure it
contains only string values.

Instead of `ClassCastException` in read phase, this pr complains in
`InstanceSpec` construction phase.

We could deprecate these constructors in next step by introducing builder
for `InstanceSpec`(#1222).

Fixes #1178, CURATOR-663.

It is hard for us to know what does
`new InstanceSpec(null, -1, -1, -1, true, -1, -1, -1)` means now.

Closes #1222.

CURATOR-495 introduced a new runSafeService field in CuratorFrameworkImpl class, and this field is
either initialized by an external ExecutorService via the builder, or it is created internally
within the class.

In the CuratorFrameworkImpl#close method though, this Executor is never closed, so the threads
that are opened by the instances are lingering there until the VM is closed by default.
Worse, if someone specifies a thread factory to the framework implementation via the builder that
produces non-daemon threads, the VM never exits due to the unstopped single thread executor.

Fixes #1277.