Determine characteristics of channel identifiers

[jrconlin]

1. Currently channel identifiers are unique UUID4 values, which are too long for many reasons. We need to determine the proper size of the entropy pool and what characters we can use to derive a valid channel ID designate.

2. For scaling, we may want to divvy things up based on endpoint pathing, (e.g. use the first character of the channel path to determine which pool/machine receives the connection). This would ensure that a given connection request returns to the same machine holding state for both sides.

[rfk]

We discussed this a little in the meeting today and decided on the following:

• Clients should not expect channel-ids to be any particular length. Rather they should expect to receive a b64url-encoded string which, when decoded, will be more than one and less than 32 bytes in length.
• We'll keep the channel ids as 32-bytes for initial version, but look at making them shorter as we come to implement the "phase 2" PAKE-based flow.