FILTER support for '*' or '?' constraint for more than 1 variable

[mitochon]

Problem
FILTER expression involving '*' or '?' does not give correct result if more than 1 variable used.

Example:
query Q1 filter "(BAR[*] < 0.1)" test.vcf -> yields no result
query Q2 filter "(BAR[*] < 0.1) & (FOO[*] >= 2)" test.vcf -> yields 3 result
Q2 is more strict than Q1 so its result should always be a subset of Q1's result.

Cause
After looking at the code, the * or ? query currently supports 1 variable.
If there are multiple variables FOO, BAR both having * or ? predicate, the index n used to evaluate BAR[n] does not get evaluated properly.
This causes incorrect results to be generated. See also TestCasesFilter#test_57 in this pull request for concrete example.

Proposed Solution
Track the current index for each query variable in FieldIterator.
In this context FOO will track some index m and BAR will track another index n.
Previously both FOO and BAR tracks the same index n even though they are at different stages in the iteration.

Verification
Added the following test files:
TestCasesFilter#test_57
test/test_filter_multiple_var.vcf

[mitochon]

This is not relevant, but I had to make this change for mvn to run successfully.

[mitochon]

Not sure about this change, but seems like this should be GENOTYPE_VAR.
Otherwise this need to be changed to

idx = FieldIterator.get().getVar(name)

Would recommend adding a test case that covers this use case.

[mitochon]

If you add this test case and undo the other changes above, EXAC_AF[*] always yields 0, so the left hand side of this expression always evaluates to true