SSLEngine closed already exception

[vgkholla]

Saw an exception in FrontendIntegrationTest during a travis build. Seems to be intermittent but filing this issue to keep track since

1. This is in the stitchBlob test (recently introduced)
2. We made some SSL changes recently

om.github.ambry.frontend.FrontendIntegrationTest > stitchedUploadTest[5] FAILED
    java.util.concurrent.ExecutionException: javax.net.ssl.SSLException: SSLEngine closed already
        at com.github.ambry.router.FutureResult.resultOrThrow(FutureResult.java:113)
        at com.github.ambry.router.FutureResult.get(FutureResult.java:99)
        at com.github.ambry.frontend.FrontendIntegrationTest.getBlobInfoAndVerify(FrontendIntegrationTest.java:836)
        at com.github.ambry.frontend.FrontendIntegrationTest.uploadDataChunksAndVerify(FrontendIntegrationTest.java:1244)
        at com.github.ambry.frontend.FrontendIntegrationTest.stitchedUploadTest(FrontendIntegrationTest.java:414)
        Caused by:
        javax.net.ssl.SSLException: SSLEngine closed already