Improve read performance approximately 2x

[mgod]

Minimize expensive calls in tight loops. This may not be worth accepting given the state of development on the sql calls right now. Tested by adding:

@property (nonatomic, readwrite) double doubleProperty;

to DecimalRecord and running this micro-benchmark:

it(@"should run some very basic performance tests", ^{
    NSDecimalNumber *acc = [NSDecimalNumber one];
    while (acc.floatValue < 10000) {
        DecimalRecord *record = [DecimalRecord newRecord];
        record.decimal = acc;
        record.doubleProperty = acc.doubleValue;
        [record save];
        acc = [acc decimalNumberByAdding:[NSDecimalNumber one]];
    }
    for(int i = 0; i < 5; i++) {
        NSDate *start = [NSDate date];
        [DecimalRecord allRecords];
        NSLog(@"Time elapsed: %f", [[NSDate date] timeIntervalSinceDate:start]);
    }
});

[AlexDenisov]

Thank you, @mgod.