Make a simple JSONRPC call before pubsub

[amaury1093]

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blockNumber$() is based on onEveryBlock$, which does a pubsub eth_subscribe('newHeads'). However, when we do blockNumber$().subscribe(), we might not get the latest block immediately, and need to wait for the next received block (can take up to 15s on mainnet).

A solution would be to make a JSONRPC call first, then do the pubsub subscribe.

Note: we were using parity_subscribe('eth_blockNumber') before, and that one returned immediately the latest block it had.

Note 2: do this on newHeads and on syncing pubsubs.

[axelchalon]

I imagine this could also be fixed upstream in the node, i.e. emit latest value (if available) on pubsub subscription