Bug: SDK overwrites application log level and example scripts fail from race conditions

[Dunsin-cyber]

1. Global Logger Mutation

The Problem: Calling Unlock() in base_client.go explicitly sets the global logrus level to DebugLevel or ErrorLevel.
The Impact: The SDK forcefully overwrites the logging configuration of any parent application importing it. Libraries should not mutate global state.

2. Race Conditions in Examples

The Problem: Example scripts (like alice_to_bob.go) do not wait for the background wallet sync to finish after calling Unlock().
The Impact: The scripts race ahead and try to spend funds before the internal database is populated.

How to Reproduce:

For this to work, #105 should have been merged to solve the panic bug. Once that is done, you can proceed with:

1. Run example/alice_to_bob/alice_to_bob.go
2. Logs after the Unlock() for alice won't show
3. It exits with error "wallet is still syncing"