use try-with-resources to simplify code that forces connections open; other cleanups

...fle.r.engine/src/com/oracle/truffle/r/engine/REngine.java

...y/src/com/oracle/truffle/r/library/utils/CountFields.java

...cle/truffle/r/nodes/builtin/base/ConnectionFunctions.java

...oracle/truffle/r/nodes/builtin/base/ForeignFunctions.java

...om/oracle/truffle/r/nodes/builtin/base/LoadFunctions.java

.../src/com/oracle/truffle/r/nodes/builtin/base/ReadDCF.java

...acle/truffle/r/nodes/builtin/base/SerializeFunctions.java

.../com/oracle/truffle/r/runtime/conn/ConnectionSupport.java

...rc/com/oracle/truffle/r/runtime/conn/FileConnections.java

...rc/com/oracle/truffle/r/runtime/conn/GZIPConnections.java

...me/src/com/oracle/truffle/r/runtime/conn/RConnection.java

.../com/oracle/truffle/r/runtime/conn/SocketConnections.java

...src/com/oracle/truffle/r/runtime/conn/StdConnections.java

...rc/com/oracle/truffle/r/runtime/conn/TextConnections.java

...src/com/oracle/truffle/r/runtime/conn/URLConnections.java