[server] Use "Impl" and "m_impl" for PImpl idiom code

We used "PrivateContext" and "m_ctx" for PImpl idiom code.
"Impl" and "m_impl" are more readable name for PImpl idiom code rather
than "PrivateContext" and "m_ctx".
[server] Use std::unique_ptr or std::auto_ptr for Impl

I didn't test with "configure --enable-cpp11". Sorry.
Merge pull request #355 from project-hatohol/use-impl-for-pimpl-idiom-code

[server] Use Impl and m_impl for PImpl idiom code.
Thank you. I confirmed that this branch can build with --enable-cpp11 option.