Don't store context in structs

[anodar]

• drop contexts from Node (this one isn't even used) and multiplexerBackend structs. (because: [0])
• move Node structure and it's functions to the beginning of the file (that way it's easier to follow what's the node's functionality and what accounts for its' state)
• rename CreateNode to New as arbnode.CreateNode is redundant and arbnode.New already implies node (more in [1])

[0] https://go.dev/blog/context-and-structs
[1] https://google.github.io/styleguide/go/decisions#package-vs-exported-symbol-name

[tsahee]

please no large restructures without an immediate reason until we have execution-separation merged.
Restructures should either be on top of execution-service-8 or wait..

[anodar]

Closing per tsahee's request #1714 (review).