fix(sdk): increase the internal channel receiver size up from 8 to 256

We suspect that there might be too many internal messages being pushed, causing a deadlock on the senders' side.
This attempts to increase the value of the buffer to give it more leeway.

Signed-off-by: Benjamin Bouvier <public@benj.me>

crates/matrix-sdk/src/sliding_sync/builder.rs

@@ -230,7 +230,7 @@ impl SlidingSyncBuilder {
 
         let mut delta_token = None;
 
-        let (internal_channel_sender, internal_channel_receiver) = channel(8);
+        let (internal_channel_sender, internal_channel_receiver) = channel(256);
 
         let mut lists = BTreeMap::new();