feat(pool): add time to mine tracking and metrics

[dancoombs]

Closes #319

Proposed Changes

• Add time to mine context to user operations in the pool
• Track the number of user operations that are currently "candidates" i.e. their fees are high enough to be included
• Track the amount of time/blocks an operation spends in the "candidate" state
• Add histogram for time/blocks to mine
• Re-use the expiry loop into a maintenance loop to limit O(n) operations
• Limitation: time to mine has a granularity of 1(s) so on fast block networks (Arbitrum) we don't have the level of granularity needed, however we do have N blocks.
• Open question: should we switch candidate time to be based on system time and not block time?

[dphilipson]

Looks great!

The base branch was changed.