[Net] Use ephemeral MARF connection on read-only Clarity call

[jcnelson]

Instead of opening a Clarity transaction to evaluate a read-only Clarity code snippet (as part of /v2/contracts/call-read and /v2/contracts/fast-call-read), the node should instead open an ephemeral MARF connection. This will not open a chainstate transaction, which ought to lead to better performance and less DB contention.