#9 Reimplement filter by index

As a convention, all single-letter (only english alphabet letters: a-z, A-Z)
key tags are expected to be indexed by relays, such that it is possible, for
example, to query or subscribe to events that reference the
event "5c83da77af1dec6d7289834998ad7aafbd9e2191396d75ec3cc27f5a77226f36" by
using the {"#e":
["5c83da77af1dec6d7289834998ad7aafbd9e2191396d75ec3cc27f5a77226f36"]} filter.

I was under the impression that only `#p` and `#e` were expected, but I was wrong. Per NIP-01, any single-letter tag is expected to be indexed. ``` As a convention, all single-letter (only english alphabet letters: a-z, A-Z) key tags are expected to be indexed by relays, such that it is possible, for example, to query or subscribe to events that reference the event "5c83da77af1dec6d7289834998ad7aafbd9e2191396d75ec3cc27f5a77226f36" by using the {"#e": ["5c83da77af1dec6d7289834998ad7aafbd9e2191396d75ec3cc27f5a77226f36"]} filter. ``` I was also wrong in implementing partial key matching in the indexes, removing a lot of code. I though this was cool for privacy reasons.