Lower channel sizes

[ss-es]

This PR lowers the channel sizes for both the internal and external event streams.

Since these channels are implemented internally as a VecDeque, we are allocating a large RawVec equal to the channel capacity on initialization. For the internal event stream this is mostly inconsequential, since the vector holds Arc references (so even an array with 100,000 entries only takes up a few megabytes).

On the other hand, for the external event stream, which holds structs like QuorumProposal, this results in a ~200+ MB virtual allocation at startup. This memory is not immediately physically allocated to the program (because it will likely exceed MMAP_THRESHOLD for the allocator), but instead becomes available as we append to the vector. Broadcasts to the channel will always append to the array in memory (because VecDeque is a circular buffer), increasing the physical memory usage. Even when we drop entries at the head of the VecDeque, we do not free any of the initial segments of memory (of course, doing this in a circular buffer would be counterproductive), so this manifests as a memory leak up until we reach the end of the underlying array at ~200 MB (or more).