Finer grained, per port tick locking

[tannewt]

Fixes #3504 hopefully.

cc @deshipu and @cwalther

[deshipu]

I just tested it with PewPew10 and it seems to fix the issue #3504, there is no flickering visible anymore.

[DavePutz]

Tested on the SAMD21 boards; Pulsein works fine and didn't see any other issues. I tested without setting RTC_READREQ_RREQ in rtc_set_continuous(), and it looks like it works fine without that bit set.

[jepler]

Glad we had the Discord discussion about this, it helped me follow what was going on better. Thanks @tannewt

The key is, this reduces the length of time that interrupts are disabled when reading the RTC. This is especially important on SAMD21 but fixes were applied to each port that potentially required them.

[cwalther]

Confirmed, the flickering is much improved on a PewPew 10.2 with the binary from CI, though not totally gone. Interestingly, the influence of the USB connection is reversed now: With a USB connection, I never get any flickering. Without a USB connection, I still get some flickering when CPU usage is low (much time spent in time.sleep()).