Migrate SDEXTENDEDTIMEOUT to QubesDB

[legoktm]

Description

In #1547 we had an experimental hack-ish to write data to qubes services to store data. At startup we do:

# EXPERIMENTAL(#1547): Check for the SDEXTENDEDTIMEOUT_N service flag and export it as
# SDEXTENDEDTIMEOUT=N.
timeout_flag_value=$(qubesdb-list /qubes-service/SDEXTENDEDTIMEOUT_)
if [ -n "$timeout_flag_value" ]; then
	echo "SDEXTENDEDTIMEOUT=$timeout_flag_value"
	export SDEXTENDEDTIMEOUT="$timeout_flag_value"
fi

Now that we have QubesDB vm-config we should switch this override mechanism to that. Also the timeout override should move to the sdk.API class instead of having MetadataSyncJob mutate it...