Remove bottleneck on chain runtime for sending queries concurrently

[romac]

Summary

By removing the bottleneck on the chain runtime, we can issue queries concurrently rather than serially as we do now, which has the potential to speed up relaying in two cases:

a) when relaying on multiple channels between two or more chains
b) when relaying between a chain A and two or more chains B, C, ... with A<->B, A<->C, ...

Note that it won't let us send multiple txs concurrently because of the monotonic constraint on the account sequence number. To speed up this aspect of the relayer, we would need multiple wallets.

---

For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate milestone (priority) applied
• Appropriate contributors tagged
• Contributor assigned/self-assigned