Self managed connections

One of the major problems with using libraries like DatabaseLibrary is managing the connection. If we implement #149, it could cause additional problems. Typically QA need to implement following logic:

Connect To Database  (...)
Query
[Teardown]    Disconnect Connection

But often Disconnect Connection is not in teardown (so the connection can remain open after test fails), we could several long running connections even if we're not querying for anything. For that reason we could add optional option for 'self managed connection' - in short, we could open and close connection each time we perform an query.

The default behaviour of the library will not change, it will be only extra option that will open/close connection if it doesn't exist.

*** Settings ***
Library    DatabaseLibrary    auto_connect=${True}
Suite Setup    Configure Connection    ${params}  # need to configure default connection

*** Test Cases ***
Test 1
    Query    SELECT * FROM table1  # will open and close connection on its own

Test 2
    Query    SELECT * FROM table2  # will open and close connection on its own