Add try-with-resources support for Entry class

[sczyh30]

Describe what this PR does / why we need it

Since 1.5.0 the min JDK version will be 1.7. We added try-with-resources support for Entry class so that it's easier to use Sentinel's API like this:

try (Entry entry = SphU.entry("someResource")) {
    // Your own logic here.
} catch (BlockException ex) {
    // Your logic for handling rejection here.
}

Does this pull request fix one issue?

Resolves #233

Describe how you did it

The Entry class now implements JDK's AutoCloseable interface so it supports try-with-resources (close method adapts to exit)

Describe how to verify it

Run the test cases.

Special notes for reviews

NONE

[CarpenterLee]

LGTM