changed term row encoding

previously we used the format:
't' <utf-8 term> <byte separator> <16-bit field id> <utf-8 docID> <byte separator>

now we have moved the field before the term, resulting in:
't' <16-bit field id> <utf-8 term> <byte separator> <utf-8 docID> <byte separator>

this means now instead of all fields with the same term being grouped together
all terms within the same field are grouped together

this allows us to enumerate the terms used with a field

this allows us to implement prefix search, and possibly improve numeric range queries