bug: fix deadlock when having multiple websocket listeners in anchor-client

[cryptopapi997]

When we create a new listener using .on, it does the following:

• Acquire a write-lock on the websocket listener
• Check if it's already initialized (and init it if not)
• Drop the write-lock
• Acquire a read-lock on the websocket listener
• Wait for events
• Drop the read-lock

This means if we want to listen to two events at once, the second listener will hang on aquiring the write-lock forever, as the first event listener will hold its read-lock until it exits. This is unneccesary, since we just need to write if the websocket listener doesn't exist yet, so a read-lock would do. This PR fixes that.

For verification of the existence of this bug, here's a quick MRE also (just run solana-test-validator in a separate terminal first and then run this & observe how it hangs forever):

fn mre() {
    let client = Client::new(Cluster::Localnet, std::rc::Rc::new(MockSigner));
    let program = client.program(Pubkey::new_unique()).unwrap();
    
    let listener1 = program
        .on::<MockEvent>(|_ctx, _event| {
            println!("Listener 1 received event");
        })
        .unwrap();
    std::thread::sleep(std::time::Duration::from_secs(1));
    
    // Will hang forever 
    let listener2 = program
        .on::<MockEvent>(|_ctx, _event| {
            println!("Listener 2 received event");
        })
        .unwrap();
    
    // Should hang forever if the deadlock is still present
    
    listener2.unsubscribe();
    listener1.unsubscribe();    
}

struct MockSigner;

impl solana_signer::Signer for MockSigner {
    fn pubkey(&self) -> Pubkey {
        Pubkey::default()
    }

    fn try_pubkey(&self) -> std::result::Result<Pubkey, SignerError> {
        Ok(Pubkey::default())
    }

    fn sign_message(&self, _message: &[u8]) -> Signature {
        Signature::default()
    }

    fn try_sign_message(
        &self,
        _message: &[u8],
    ) -> std::result::Result<Signature, SignerError> {
        Ok(Signature::default())
    }

    fn is_interactive(&self) -> bool {
        false
    }

}

[vercel]

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[cryptopapi997]

wdyt @jamie-osec