Function Registries, and enforce function existence errors

[araddon]

Currently the function-registry is assumed to be global scope. This is flawed, each table in a schema would have its own function registry, so that a elasticsearch backed table would not have the same available functions as say a mysql one, etc.

• [ ] no longer fall back to global-scope functions, use only tree-specific registry: https://github.com/araddon/qlbridge/blob/f23639094de83317243698fe77d269f387147712/expr/parse.go#L765
• [ ] raise errors on missing functions always https://github.com/araddon/qlbridge/blob/f23639094de83317243698fe77d269f387147712/expr/parse.go#L643