Fix ReadSTB Timing

[esarver]

This fixes the issue of calling ReadSTB() every 2ms when connected to an instrument over a VISA connection.

Now we only call ReadSTB() after the user sends something to the instrument (such as a TSP command, a script, or an upgrade) OR as a heartbeat every 3 seconds in the case an instrument stops responding.

[github-actions]

Package	Line Rate	Health
kic-discover-visa/src/ethernet/mod.rs	0%	❌
kic-discover/src/lib.rs	0%	❌
kic/src/main.rs	0%	❌
kic-discover/src/instrument_discovery/mod.rs	0%	❌
kic-discover-visa/src/main.rs	0%	❌
kic-visa/src/process.rs	0%	❌
instrument-repl/src/resources/mod.rs	100%	✔
kic-discover-visa/src/instrument_discovery/mod.rs	0%	❌
instrument-repl/src/instrument.rs	86%	✔
kic-discover/src/ethernet/mod.rs	0%	❌
instrument-repl/src/state_machine.rs	63%	✔
kic-discover-visa/src/lib.rs	0%	❌
kic/src/process.rs	0%	❌
instrument-repl/src/tsp_error.rs	0%	❌
kic-visa/src/main.rs	0%	❌
kic-discover/src/main.rs	0%	❌
kic-discover-visa/src/visa.rs	0%	❌
instrument-repl/src/repl.rs	0%	❌
kic-discover/src/process.rs	0%	❌
Summary	11% (295 / 2579)	❌