Ion 1.1 managed writer should flush and reset buffers when a symbol table is manually written

[tgregg]

When the user (or, more likely, a system reader via writeValues) writes a symbol table or IVM, the writer should flush any previously buffered data to the output. This is a performance optimization, and should allow parity with Ion 1.0. Here's some data to support that, produced using the ion-java-benchmark-cli, which writes data using a system reader provided to IonWriter.writeValues.

Ion 1.0:
194 ms / op

Ion 1.1 - all symbols interned
710 ms / op

Ion 1.1 - all symbols inline
234 ms / op

Ion 1.1 - all symbols interned - forced flush every 500 values
175 ms / op

Ion 1.1 - all symbols inline - forced flush every 500 values
229 ms / op

[tgregg]

Update: we should still make sure this is being done, but it may not be the cause of the performance discrepancies noted. The benchmark CLI isn't currently doing a system write during the timed portion of the write benchmark; only during the one-time conversion performed before generating the write instructions or for read benchmarks that provide a different format as input. More investigation needed.