proc/gdbserial: Reload thread registers on demand
proc/gdbserial: Reload thread registers on demand

Instead of reloading the registers for every thread every time the
process executes, reload the registers on demand for individual threads
and memoize the result.

go-delve/delve,aarzilli/delve,aarzilli/delve,derekparker/delve,aarzilli/delve,go-delve/delve,go-delve/delve,aarzilli/delve,derekparker/delve,go-delve/delve,go-delve/delve,derekparker/delve