Add __slots__ to replication commands.

[clokep]

I'm hoping this will slim down our replication needs very very slightly, but it is also minimal effort.

WIth this change we have the following, sizes are done using _get_size_of.

Class	Original size (bytes)	Final size (bytes)	Savings (bytes)
Any _SimpleCommand subclass	408	360	48
RdataCommand	592	384	208
PositionCommand	672	448	224
UserSyncCommand	728	456	272
ClearUserSyncsCommand	416	360	56
FederationAckCommand	504	400	104
UserIpCommand	752	432	320
LockReleasedCommand	544	376	168

Code to produce the above

```python from synapse.util.caches.lrucache import _get_size_of from synapse.replication.tcp.commands import * def do(inst): print(f"{inst.__class__.__name__}: {_get_size_of(inst)}") do(ServerCommand("")) do(RdataCommand("", "", None, ())) do(PositionCommand("", "", 0, 1)) do(UserSyncCommand("", "", None, True, 1)) do(ClearUserSyncsCommand("")) do(FederationAckCommand("", 1)) do(UserIpCommand("", "", 1, "", None, 1)) do(LockReleasedCommand("", "", "")) ```

[clokep]

(Aside: I was slightly surprised these classes aren't defined with attrs or stdlib dataclasses, which would make it easier to turn on __slots__.)

Due to the NAME class var I was having trouble getting that to work and decided it wasn't worth my effort figuring out how to make those compatible.

[clokep]

Seems reasonable. I guess this only helps a lot if we have a huge queue of replication commands to work through?

It should help just in general with allocating less memory.