Remove linear search for unused positions when queuing telemetry
updates; instead, track available positions such that insertion
is more performant at a modest cost (bound to the number of subscriptions)
of retrieval. Additionally, add implementation notes in-line.
WTD-1202.
Implement data structures which support lossless/lossy
behaviors for telemetry subscriptions (that is, allow
users of the telemetrySubscriber to decide whether or not
they are willing to drop updates in favor of only being
notified with the latest available values.) WTD-784.
Add specs for data structures which will support resolution
of WTD-784 by retaining all data not yet received by callbacks
(instead of just the very latest) such that plots can ensure
they do not miss streaming data.
