Split Conn.querySystemPeers

[dkropachev]

In order to make internals testable we need to split to code from Conn.querySystemPeers into query execution and decision on what to execute, at the end we should end up with:

1. isSchemaV2 flag on Conn that is initiated on Conn during Conn.init:
   Right now isSchemaV2 is set in querySystemPeers, let's move it to init and do it once.
   Please check that init() is called on reconnect, to ensure that when/if node is updated, driver rereads it as well.
2. Function Conn.querySystem() that Conn.session.usingTimeoutClause into consideration when executes a query.
3. Standalone function that takes Conn (as interface) and returns same result as querySystemPeers

[sylwiaszunejko]

@dkropachev I believe it was fixed with #368