fix: add subscription mapping lock

[emizzle]

Lock access to the subscriptionMapping table in the subscriptions module. This needs to be done in the getChanges exception handling when a new filter is created (on filter not found), and in the unsubscribe functions, at a minimum.

Create a test scenario where a memory leak would happen such that:

1. the filter not found error condition is created`
2. When eth_newFilter is called, and control is given to another function in the event loop (the test), unsubscribe is called which will attempt to delete a filter id that doesn't yet exist (since async control hadn't returned from the eth_newFilter call yet).