Allow pvws configuration to be set at runtime

[GregJHarris]

• store is now a function that takes a configuration argument and returns an instance of the store.
• The store instance is a singleton, multiple calls to store will return the same instance.
• Runtime configuration available for PVWS_SOCKET, PVWS_SSL, THROTTLE_PERIOD.
• Runtime configuration overrides the environment variables.
• Bumped package version to 0.9 as this is a breaking change

[abigailalexander]

Tested this and it works well, thanks