Improve managing connections to peers

[matjazv]

What was the problem?

This PR resolves #63.

How was it solved?

• If the number of connections exceeds the maximum allowed number of connections, some of those connections are terminated.
• If the number of connections drops below the minimum number of connections, p2p tries to connect to some of the known peers.
• To prevent an indefinite amount of time to be connected with any of non-fixed peers, connections are randomly closed at predefined time intervals.
• Some unit tests were refactored to not use busy wait anymore.

How was it tested?

• New unit test was implemented.
• Existing and new unit tests passed.