Fix context inheritance and optionally add context to global loggers

[christiand93]

Changes

• Fix context inheritance
  • Child loggers inherit context fields (e.g., correlation_id) from their parent logger, if present.

• Add context to global child loggers

  • Child loggers derived from the global logger optionally support context features now. This includes
    • support for setContextProperty(), setCorrelationId(), setTenantId(), setTenantSubdomain() and the respective get methods.
    • auto-generated correlation_id (default configuration)
    • context inheritance

  • A new child logger can be equipped with a new context by specifying true for the createNewContext parameter:

    const logger = log.createLogger(null, true)

• Add a boolean return value for setContextProperty(), setCorrelationId(), setTenantId() and setTenantSubdomain() to indicate if the set operation was successful. If a logger does not have a context, the fields cannot be set and the methods return false.
• Add test cases for context inheritance
• Add/update documentation on contexts and child loggers

Fixes #185