Implement client context

[kravets-levko]

This library is designed so we have to initialize a root object (DBSQLClient) which later produces many of other object which represent sessions, operations, results sets and so on. And most of those objects require some common stuff, for example - logger, driver, connection - which are created and owned by their parent DBSQLClient instance. Previously we were passing those objects separately down the whole tree, and with time it became harder to add new items to that common context. In this PR, common objects are wrapped together into a context provider (implemented by DBSQLClient) which is then passed to all classes that need it.

This PR is also a spin-off of #174 (#193) which itself is an example why these changes are needed. There we need to access a HttpConnection instance owned by DBSQLClient in DBSQLSession and CloudFetchResult which required to pass this object down the tree.

Other use cases for this (not implemented in this PR, but planned for future) - add logging to classes which weren't capable to write to log until now (result handlers), and also move global config settings to context.

It may be easier to review changes commit by commit. Tests were just updated to match new signatures, without changes to their logic

[susodapop]

Very good PR description. I've evaluated each commit sequentially. I'm not super-familiar with Node best practices but nothing here sticks out as a bad pattern and the changes make sense. This is good to merge assuming existing tests pass.