Speed up qp problem encoding

[randomir]

ASV benchmarks on my machine:

• active_qubits(): 34 ms -> 22 ms
• active_qubits() on BQM: 156 ms -> 121ms
• encode_problem_as_qp(): 103 ms -> 75 ms
• encode_problem_as_qp() on BQM: 650 ms -> 560 ms and then -> 400 ms when converted to dict upfront

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.86%. Comparing base (5597ff0) to head (91b662e). Report is 55 commits behind head on master.

Files with missing lines	Patch %	Lines
dwave/cloud/coders.py	75.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #661 +/- ## ========================================== - Coverage 88.91% 88.86% -0.06% ========================================== Files 42 42 Lines 4810 4815 +5 ========================================== + Hits 4277 4279 +2 - Misses 533 536 +3 ```

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