ec3po: Update threadproc_shim.py to use threading-oriented implementations.
ec3po: Update threadproc_shim.py to use threading-oriented implementations.

This migrates ec3po from using subprocesses to threads.

BRANCH=none
BUG=b:79684405
CQ-DEPEND=CL:1279118, CL:1281997, CL:1282265
TEST=With a servo_micro connected to an octopus_ite, functionality
involving the EC console continues to work.  I tested dut-control
ec_uart_pty, servo_console_pty (both tested with minicom), dut_i2c_mux,
enable_ite_dfu, get_ite_chipid commands.  Additionally, servod shutdown via
either ctrl+c or SIGTERM still happens correctly, without any delay,
leftover processes, or tracebacks.

Change-Id: Ib59aa83400ee982a5cc4d1d9d1609197a1145514
Signed-off-by: Matthew Blecker <d18f4466f2802f9c78c5c14a42b8c21fdbfabfcd@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1279145
Tested-by: Ruben Rodriguez Buchillon <12b0357cd989b603267f0b7cb81c747471f94bd1@chromium.org>
Reviewed-by: Ruben Rodriguez Buchillon <12b0357cd989b603267f0b7cb81c747471f94bd1@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec