Serialization support of fieldpaths not comprehensive

[jpbetz]

During review of #292, a few pre-existing gaps in serialization support were noticed that we should really fix:

• Overflows are possible using the value interface's IsInt/AsInt. This could happen in an aggregated API server when the writer is proto and the value type is uint64, for example.
• Custom Marshalers (JSON or CBOR) not supported in any of the valueReflect (for aggregated, or in-tree for Quanity/Time for map key values)

Also there is this potential optimization:

• In serialize.go, we could optimize for "out of sorted order".

[jpbetz]

cc @liggitt @benluddy @deads2k @lalitc375

[jpbetz]

cc @inteon