Skip hash bucketing for small partitions

[rkenmi]

This PR adds a feature to skip hash bucketing if the user-supplied hash bucket count is set to 1.

Changes

• If hash bucket count is set to 1, delta file envelopes are loaded into the merge step directly to compact tables
• Since the merge step could be dealing with delta file envelope object refs (remote) or pure delta file envelopes (local), an abstraction layer is added for delta file envelope retrieval during invoke by merge.

Initial Benchmarks

Input Delta Size	Default Run 1 (secs)	Default Run 2 (secs)	Default Run 3 (secs)	Default Avg. (secs)	Skip HB Run 1 (secs)	Skip HB Run 2 (secs)	Skip HB Run 3 (secs)	Skip HB Avg. (secs)	Skip HB % Improvement
1.2 GB	196.80	194.44	194.91	195.38	80.54	79.88	78.95	79.79	59.16%
1.7 GB	231.34	215.52	251.69	232.85	117.79	113.73	117.22	116.25	50.08%
3.5 GB	259.40	304.18	264.16	275.91	249.14	257.83	231.42	246.13	10.79%
6.5 GB	377.45	349.44	336.23	354.37	377.27	369.76	305.44	350.82	1%

[raghumdani]

Functionality wise, looks good. I think a bit of refactoring is needed to ensure it's easier to maintain this code in future.

[raghumdani]

Looks good mostly. Couple of minor comments.

[raghumdani]

LGTM!