1. Modified vcf_parse to update rlen only when an END INFO tag is seen -

Issue: https://github.com/broadinstitute/gamgee/issues/191
1. Modified vcf_parse to update rlen only when an END INFO tag is seen -the END tag is NOT encoded into v->shared.s
2. Modified vcf_write to add END INFO tag if required

Gamgee has a function `set_alignment_stop(uint32_t)` that does the right thing. We will warn the user about setting end as an info field, but we don't want to pay the cost of always checking for END when setting info fields. Also gamgee bypasses this function as the `VariantBuilder` constructs the byte arrays on its own.