EVM-687 Dial queue slots

[igorcrevar]

Description

slots concept in order to limit maximal outbound connections:

• func (s *Server) runDial() waits for something to be inserted into the queue
• after task has been poped from the queue, it tries to take one slot for new outbound connection. If there are no available slots the routine is blocked
• whenever disconnect/failed to connect events occurs, one slot is released and becomes available

This task is necessary in order to finish EVM-543

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Breaking changes

Please complete this section if any breaking changes have been made, otherwise delete it

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

Manual tests

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Documentation update

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Additional comments

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