Combine binding contexts to speedup hook execution on high loads

[diafour]

1. If queue contains a sequence of HookRun tasks for one hook, then this hook can be executed with combined binding context.
2. Binding context is already an array, so hook should iterate over it, not just get first element.
3. A pack of binding contexts with similar skipKey can be compacted, so only the latest binding context will remain.

[diafour]

1 and 2 are implemented in #144

3 is implemented in #150:
skipKey renamed to group. A pack of binding context with similar group is compacted to one binding context. This compacted context contains only "binding" and "snapshots" fields.
group and includeSnapshotsFrom keys are mutually exclusive.
If group is defined, then includeSnapshotsFrom is filled with all kubernetes bindings in this group.