Enable pickling of schema registry client by updating `_SchemaCache`

[jkleung11]

• Mirrors PR 56 in python-schema-registry-client
• Attempted to use AvroSerialzer and AvroDeserializer within pyspark udfs to (un)serialize contents of a dataframe but ran into TypeError: cannot pickle '_thread.lock' object
• The serializers are dependent on SchemaRegistryClient, which in turn uses _SchemaCache
• Added __setstate__ and __getstate__ methods to process the .lock attribute of _SchemaCache

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
0 out of 2 committers have signed the CLA.

:x: Jonathan Leung
:x: jkleung11

---

Jonathan Leung seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}