Improve observers codebase

[swift1337]

Describe the Issue

• Move all WithZetacoreClient, With* methods to constructor. Make API as concise as possible
• Simplify methods ReadLastBlockScannedFromDB, SaveLastBlockScanned, WithLastBlockScanned, WriteLastBlockScannedToDB
• don't use global metrics

Expected Outcome
Observers codebase should become way cleaner and simpler

[lumtis]

Added it to #1611

We can use this EPIC to track all issues for improving ZetaClient codebase quality.

In some point we can consider closing the epic and just using triaging with code-qualidty label but for now it allows to have a high-level view on the consideration for most important code quality improvement