Bugs associated with searching

[tdb01r]

#3358, #3229, #3192, #3191, #3079, #3129, #3160, #2995, #2245, #2032, #1662, #888

Concrete problems:

• Searching code is obscure/lacking documentation
• Username searches don't work for names less than 3 chars long
• Advanced searches for hyphenated terms ("funky-town") are broken (either 'funky' or 'town' depending on hash indexes)
• Simple search only applies text-based searches (would be nice to also EQuals match the entire query for each field?)
• Empty terms aren't cleaned up (DELETE * FROM __index WHERE ids=':')

Other stuff:

• Could the grep tables be rolled into the normal index table? (Reduce db complexity)
• More efficient mechanism than colon-concatenated? e.g. bit-vector or packed ULLs?