node: simplify start node cmd

[iurii-ssv]

Rebased onto #1819 (only 1 last commit in this PR is new, throw away the rest because it will be impossible to rebase once #1819 is squash-merged)

Run function of StartNodeCmd is a bit hard to navigate (due to it's size), plus it's poorly structured - this PR tries to simplify it by:

• grouping related code together (where applicable)
• splitting off pieces of Run function into separate helper-funcs

this PR is in Draft for now - it depends on the following PRs to be merged first:

• node: startup cleanup plus enable jemalloc #1819
• alan: cleanup after fork #1820

(notes to self) after that ^ finish the following sub-tasks:

• (where applicable) group initialization of related cfg settings together
• move blocks of code out of StartNodeCmd.Run into separate funcs in todo.go
• split todo.go into appropriately named files

Before merging:

• test on stage