telemetry: improve query/log interface

followup to https://github.com/aws/aws-toolkit-vscode/pull/2650

## Problem

1. TelemetryService.findIter() is unstable
2. TelemetryLogger never flushes its items, and queries don't have a way to limit searches to time intervals

## Proposal

1. Eliminate either TelemetryService.findIter() or TelemetryLogger, or both (and add a stable query interface to TelemetryService)
2. add options to the telemetry logger so that components can "mark" a time they are interested in, or just flush (clear) the whole thing (then we don't need the "marker" concept). And/or add a way to query by a time range.

Start by reading PR #2650 and the existing TelemetryService.findIter() and TelemetryLogger entry points. Compare their current query and flush behavior with the reported instability, missing flushes, and lack of time-range limits. Done requires a settled interface direction with behavior for querying, flushing or marking, and time-range limits.

Written by the indexing model from the issue text.