Investigate support for queries using XAPI-style predicates

[jkoshy]

XAPI offers the ability to query the map using complex predicates.

For example:

• /api/0.6/node[amenity=hospital]
  Queries the map for all nodes containing <tag k="amenity" v="hospital" />
• /api/0.6/*[key1=value1][key2=value2]...
  Queries the map for all elements matching the predicates (key1 == value1 && key2 == value2).
• /api/0.6/relation[not(way)]
  Retrieve relations that do not have way members.
• /api/0.6/node[@user=name]
  Retrieve nodes last change by a specific user name.

Investigate how best these queries could be implemented in the current architecture.