Use a unified `Context` for ZetaClient

[lumtis]

We use context.Background and context.TODO throughout the code.

This might become a problem if a Go routine need to be canceled when another is canceled, or if ZetaClient is incorporated in another package.

It is also in general proper to determine determine a unified context that is used in the package.

Solution:

• Add a context object Ctx as a field of the different object: CoreObserver, ZetaCoreBridge, Signer, etc...
• Replace context.Background with these values
• Initialize with context.Background the root context

[swift1337]

I think the root cause of the issue is that currently we don't pass ctx context.Context as the first argument to all functions that do i/o / networking / etc. I'll take care of this

[lumtis]

I think the root cause of the issue is that currently we don't pass ctx context.Context as the first argument to all functions that do i/o / networking / etc. I'll take care of this

Sounds good

[lumtis]

Closed by #2428