use base64+zstd for account data encoding by default, add ping support and make recv cancellation safe

[dovahcrow]

Sorry I squeezed the fixes for #17 #19 and a base64+zstd fix into the same PR.

The first commit in the PR fixes the following problem.
The jsonParsed encoding won't work if the account data can be parsed by the RPC node. For example, the spl-token account will be actually parsed! The notification from WS becomes

{
    "jsonrpc": "2.0",
    "method": "accountNotification",
    "params": {
        "result": {
            "context": {
                "slot": 114915064
            },
            "value": {
                "lamports": 2039280,
                "data": {
                    "program": "spl-token",
                    "parsed": {
                        "info": {
                            "isNative": false,
                            "mint": "USDhTjkUXFfigLELiFpbBnpLmEm4aXHvdY2kDSadJDH",
                            "owner": "8dyR4YwFHzh2Pxmgs9TPst5Mu4kkES5tu8w6SuhdEyUG",
                            "state": "initialized",
                            "tokenAmount": {
                                "amount": "5771086231612606",
                                "decimals": 9,
                                "uiAmount": 5771086.231612606,
                                "uiAmountString": "5771086.231612606"
                            }
                        },
                        "type": "account"
                    },
                    "space": 165
                },
                "owner": "TokenkegQfeZyiNwAJbNbGKPFXCWuBvf9Ss623VQ5DA",
                "executable": false,
                "rentEpoch": 266
            }
        },
        "subscription": 10447
    }
}

and cannot be deserialized into the AccountRepresentation struct.

Choosing base64+zstd because it is compressed and should save network bandwidth theoretically (which is the bottleneck).

The rest two commits fixes #17 and #19.

[y2kappa]

@boymaas @karim-agha

[y2kappa]

@dovahcrow let us know when it’s ready

[dovahcrow]

@dovahcrow let us know when it’s ready

It's ready.

[karim-agha]

Not a very thorough review on my part, but lgtm on first sight. Maybe it's good to have one more pair of eyes looking at it.

[y2kappa]

@boymaas @oeble ping

[boymaas]

Looks good, a keep alive and base64+zstd. Approved from my part

[y2kappa]

@dovahcrow

[y2kappa]

please bump the version

[dovahcrow]

done