Allow running Synapse with the asyncioreactor enabled

[sandhose]

This adds a flag --asyncio-reactor, which sets the SYNAPSE_ASYNCIO_REACTOR environment variable to 1.
It also adds a ASYNCIO_REACTOR environment variable to the Docker image to set this flag.

Note that it currently also sets SYNAPSE_ASYNCIO_REACTOR to 0 when the flag is not set, which currently enables the asyncioreactor anyway, so this will require some changes in Synapse first: matrix-org/synapse#14092

Related: matrix-org/synapse#14091

[DMRobertson]

It seems a bit odd to accept an env var ASYNCIO_REACTOR, convert that to a command-line flag --asyncio-reactor and then use the flag to set a different env var SYNAPSE_ASYNC_IO_REACTOR. Does SYNAPSE_ASYNC_IO_REACTOR=1 synapse_sytest.sh pass through the environment var to the synapse processes?

Well, I took a quick look at the perl and I have no idea if the environment will pass through when we spawn the process.

sytest/lib/SyTest/Homeserver/ProcessManager.pm

Lines 98 to 103 in 4fa32d0

	my $proc = IO::Async::Process->new(
	%process_params,
	stdout => { on_read => $on_output },
	stderr => { on_read => $on_output },
	on_finish => $self->_capture_weakself( '_on_finish' ),
	);

[DMRobertson]

In the interests of getting asyncio tested sooner rather than later, let's roll with this. Note that matrix-org/synapse#14092 has already landed.