Remove immediate space pointers from VDBE

[kyukhin]

In order to make VDBE portable and (possibly) to improve DDL performance,
let's remove immediate space pointers from VDBE.
This can be done as following:

• Fix virtual registers range to hold pointers to systems spaces. While
  emitting code, keep track which system spaces are in use.
• While emitting VDBE code, use free registers to refer to spaces,
  keep track, which registers refer to which spaces
• At VDBE prologue: initialise those system spaces pointers, which
  are used in the code.
• Also in prologue. Load space pointers into registers allocated during
  code emit.

Part of (or even supersedes) #3319