Schema lookups for spaces and indexes

[tehmaze]

This allows one to query available space and index names from the schema, given that the user has read access to the _space and _index spaces.

[mialinx]

Excellent!

But we have expected a bit different approach.
Please look at:
#7
#8

It would be greate if you could add just additional field (or method) called Schema to connection structure. Something like that:

type Field struct {
    Name string
    Type string
  }

  type Space struct {
    Id        uint
    Name      string
    Type      string
    Temporary bool
    Fields    []*Field
    Indexes   []*Index
  }

  type Index struct {
    Name   string
    Type   string
    Unique bool
    Fields []*Field
  }

  type Schema struct {
    Spaces  []*Space
    Version uint
  }

  // conn.Schema.Spaces // []*Space
  // conn.Schema.Spaces[1].Indexes  // []*Index
  // conn.Schema.Spaces[1].Fields // []*Field
  // conn.Schema.Space('spacename') // *Space
  // conn.Schema.Space('spacename').Index('indexname') // *Index

conn.Schema field should be populated during connection and atomically updated when schema version number changes (see #7)

[mialinx]

Implemented in b516184