`check_signer` bug

[asalzmann]

It seems the underlying C library in check_signer is sharing global state. Observe what happens when I create two separate SignerClient (different addresses / api keys, same api key index)

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TEST 2: SEQUENTIAL CREATION (Alice first)
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1. Creating Alice's SignerClient (account 221934, api_key_index 2)
   Alice's check_client: Success

2. Creating Bob's SignerClient (account 295954, api_key_index 2)
   Bob's check_client: Success

3. Re-checking Alices client after Bob's creation
   Alice's re-check: accountIndex does not match. expected 295954 but got 221934 on api key 2

reproduce with the following script

simply replace sys.path, api keys, addresses at the top
https://gist.github.com/asalzmann/6c76cdd0d5c3717b2712e0dbf7aecdea