Debug Query as SQL string.

[pravic]

Summary

Allow to print the current SQL query, whether it's bound or not. Useful for logging and diagnostics.

Note: I chose Debug over Display because the latter is too implicit and somebody might accidentally pass a query into a string.

Albeit, something like fn Query::into_string(self) -> String can be useful on its own - as a poor man's query builder:

let q = client.query("SELECT id from system.users WHERE name = ?");
let q = q.bind("default");

// store the query string for later 
let query_string = q.into_string(); // returns `q.sql.finish()` inernally

// then execute it:
client.query(&query_string).fetch();

closes #146

Checklist

Delete items not relevant to your PR:

• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided so that we can include it in CHANGELOG later
• For significant changes, documentation in README and https://github.com/ClickHouse/clickhouse-docs was updated with further explanations or tutorials
• conflicts with Query parser: encode ?? as ? #154, this PR needs to be rebased on top of that

[loyd]

Only misleading Debug impl, all other changes are fine

[CLAassistant]

All committers have signed the CLA.

[loyd]

@pravic, thanks for your contribution!