Investigate if decimal context can be set to match spanner numeric limits

[vi3k6i5]

Since decimalField/numeric support was added in #620 some tests were skipped. One of those tests "db_functions.math.test_degrees.DegreesTests.test_decimal", is failing because spanner numeric fields really aren't precise enough. Computed server-side, spanner's "degrees" isn't almostEqual to Decimal(math.degrees(...)). But what if we changed the precision in the decimal context?

Investigate if we can change the decimal context, so that the precision in the context matches spanner's numeric scale limit before doing any numeric ops.