load: identify choke point in signalc send path

context

with #201 (closed), we demonstrated that signalc can handle sending 1,000 messages with significantly higher throughput than signald
however, it still takes roughly 20sec to transmit all 1,000 messages (vs. an acceptable 2 sec for 100 messages)
in this card, we would like to identify the point(s) in the execution flow

map out the execution of handling a send request from signalboost into stages (from receiving the message off the wire, to sending a response back) in intuitive language that helps us reason about throughput and flow (do "domain modeling" for our application's performance)
measure the time spent waiting in queue at each stage in this flow, find the part of the flow where messages stack up (and spend a long time waiting in the queue) -> this is the chokepoint
optionally: corroborate the above measurement by measuring throughput at various phases of the flow (with the caveat that throughput will go down throughout the flow once a backup has started at the chokepoint --> on its own this measure will not help us identify the chokepoint, but will perhaps be useful for helping identify when it begins)

Edited Mar 30, 2021 by aguestuser