Annex introducing definition for Proxy

[plebhash]

Originally on #98 I added a section to the Mining Protocol specs introducing a formal definition of Proxy.

I felt that was necessary, since there's multiple mentions to the term Proxy on the Mining Protocol specs, without a proper definition, which feels like a chicken-and-egg kind of problem.

However, @jakubtrnka feedback was that this kind of definition deserves a separate annex, given that this is a kind of application of the protocol.

#98 (comment)

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Below are some different proxy definitions suggested by @jakubtrnka :

Multiplexing proxy

• minimalistic design, minimum or even zero configuration options
• simple passthrough-proxy for extended channels.
• multiplexes standard channels through a single upstream connection via group channels
• no difficulty/target management - everything managed via upstream server
• only manages group, channels, ids, exranonce-prefixes. Merges and dispatches messages coming from downstream or upstream respectively. Uses channel_id based logic.

Demultiplexing proxy

• generic application - mainly for demonstration purposes
• simple pass-through for extended channels
• groups downstream multiplexed standard channels into groups based on a specific criteria - either username, device-type etc... and splits them into separate upstream connections
• this app would typically be used together with multiplexing proxy
  organizes different groups using SetGroupChannel

Aggregation proxy

• logically separates down- and upstream sides
• maintains only extended upstream channels - main purpose is to reduce number of mining sessions by aggregation
• splits hashing space of the upstream job and assigns multiple smaller extended channels or standard channels to a given mining job.
• does difficulty/target management
• handles user identities

Translator proxy

V2 to V1 down-to-up translation

• zero configuration trivial translation layer for sv2-native miners and legacy sv1 pools

V1 to V2 down-to-up translation

• trivial translation layer for legacy sv1 miners and sv2 pool
• there is a single configuration option: whether a standard or an extended channel should be opened.