getRecentPerformanceSamples(): Filter to return results since a certain slot only

[ilya-bobyr]

Problem

Performance stats are computed by each node, based on the node own performance.
For nodes that are running in sync with the rest of the network, node stats match network stats close enough.
But when a node is catching up with the rest of the network, it might be reporting stats that could deviate from the rest of the network performance.

For example, the number of transactions is computed based on the number of transactions in blocks a given node has processed in the last 60 seconds.
When the node is running in sync with the rest of the network, this number will show the network performance.
When the node is catching up, this number will show the node performance.

Ideally, it would be great to show both numbers at all times: one set of stats for the network, and one for the node.
But, for now, we only report the node stats.

If one wants to monitor specific a given node, they are in luck.
If one wants to see the network performance, they need a node that is in sync.

This is a follow-up to a discussion in #29159.
Specifically these comments:

• Make getRecentPerformanceSamples() return the number of vote and non-vote transactions  #29159 (comment)
• Make getRecentPerformanceSamples() return the number of vote and non-vote transactions  #29159 (comment)

Proposed Solution

Add minContextSlot to getRecentPerformanceSamples().
It specifies a minimum slot that the node have already processed, in order to return a successful response.
By choosing the target slot to be close to the tip of the chain, the caller can ensure that the node is reasonably up-to-date.