Support Embulk >0.8

[yzp0n]

Hey. Thanks for publicizing this great piece of software! We are happily using embulk-output-orc in our production pipeline.

We found an incompatibility issue with Embulk >0.8 which enabled "Page Scattering"[release note] by default.
It seems the ScatterExecutor instantiates multiple instances of the plugin and end up running orc.Writer in parallel, while the default orc.MemoryManagerImpl is not thread-safe [code].
With Embulk >0.8, we get a lot of "Wrong Owner Thread" warnings [code], and random crashes when finalizing the write.

If you are open to contributions, we are happy to collaborate on a fix. We have a tentative fix which seems to be working on our prod, which we'll open a PR shortly.

(突然の Issue 失礼いたします。日本語のやりとりを希望される場合は、お申し付けください。)

[yuokada]

@yzp0n Thanks you for your contribution. I merged your PR.

[yzp0n]

Thank you for the merge! Let me close this issue as resolved.