[ENH] Efficient TS predict

[paxcema]

Changelog:

Runtime

• Refactored helpers.ts.get_inferred_timestamps() to strictly use vectorized operations.
• Refactored mixer.nhits.__call__() to strictly use vectorized operations.

Other

• Improved predictor time logging: @timed becomes @timed_predictor, and we introduce timed for logging arbitrary calls to the logger's stream.
• Added timed call to the ensemble section for DEBUG levels.