Provide an option to maintain a global query server connection

[jeongsoolee09]

Currently, the query server connection variable, codeql--query-server is a buffer-local variable. This works when developing self-contained .ql file. However, when the query file gets larger and it makes sense to delegate some of the helper classes to query library files (.qll) files, then having a shared database server connection across the .ql file and the .qll files makes it convenient to develop them.

There are potentially many ways to implement this:

• Add a transient suffix command to inherit a database server connection from another buffer
• Make the query server connection global for all buffers, even for the future ones (This is what the Visual Studio Code extension does)