yaml: support buffer recycling that doesn't need ThreadLocal

[pjfanning]

https://github.com/search?q=repo%3AFasterXML%2Fjackson-dataformats-text+ThreadLocal+path%3A*.java&type=code

Raised for discussion. I'm happy that ThreadLocals are ok for most users.

[cowtowncoder]

Would require quite a bit of work -- might be best to start by exposing a YAMLFactory.Feature that allows disabling buffer recycling for UTF-8 reader/writer (but doesn't affect jackson-core based internal processing buffer recycling).