rpcdaemon: extract awaitable async task run by executor

[canepat]

This PR introduces a common async_task primitive that can be used to execute a task on some executor and co-await asynchronously for its completion. Typical usage: any I/O thread delegates long-running tasks to a thread pool by calling a coroutine that starts the work and is suspended until completion.

The key factors for introducing this abstraction are:

• implementation of asio::async_compose + asio::post pattern in one place
• complete exception handling (missing in many current usages)
• avoid code duplication
• improve readability at the call site

The following modules throughout the RPC daemon has been refactored to benefit from async_task:

• http::Connection
• evm_debug
• emv_trace
• call_many
• estimate_gas_oracle
• evm_executor