Add metadata to TopicPartition

[milindl]

This PR builds on #1126 (retains commit history).

Since there are comments on the prior PR, I'll both add those comments here and address them here.

[CLAassistant]

All committers have signed the CLA.

[milindl]

Updated PR to use strings as the metadata type. Reading with "s" ensures that we get a utf-8 encoded version of the unicode object or string, as well as an error for a NULL character in the middle of the string, which is what we want.