Feature: Simplify SPLIT KEY RANGE

[Denchick]

Problem: Non-obvious syntax and need to calculate boundaries manually.

-- Current (unclear that new_id is the NEW range):
SPLIT KEY RANGE key_range3 FROM key_range1 BY 1073741824;

-- Proposed — explicit parameter names:
SPLIT key_range1
  INTO key_range1_left, key_range1_right
  AT 1073741824;

-- Or with automatic midpoint calculation:
SPLIT key_range1 IN HALF;
SPLIT key_range1 INTO 4 PARTS;

Related to: #1887

[Denchick]

Same for SPLIT KEY RANGE: #1967 (comment)