Prevent ConcurrentModificationException thrown from Metadata class

[fractalwrench]

Goal

Prevents a ConcurrentModificationException from being thrown in the internally used MetaData class.

MetaData does correctly use a ConcurrentHashMap as the default value for the store property, but within the copy() method supplies a regular HashMap. Whenever client.notify() is invoked the global metadata state is copied, which can result in a ConcurrentModificationException being thrown in subsequent edits.

This changeset resolves the problem by enforcing that store must be a ConcurrentHashMap.

Testing

Added a unit test which throws an exception without the changes, and confirmed that it no longer throws an exception.

[bugsnagbot]

Android notifier sizes

Format	Size impact of Bugsnag (kB)	Size impact of Bugsnag when Minified (kB)
APK	1437.78	1358.79
arm64_v8a	369.24	287.33
armeabi	348.77	266.85
armeabi_v7a	328.29	250.47
x86	406.1	324.19
x86_64	389.72	307.81

Generated by 🚫 Danger