[Gzz] Global cell IDs

[B. Fallenstein]

Hi!

I was thinking about global cell ids. So far they contain the id of the 
mediaserver block in which they were created-- which is difficult, 
because we need to mention them in that mediaserver block *before we 
know the block's id*.

It gets harder: think about different slices refering to each other. If 
we create a cell X in slice A and a cell Y in slice B, and then 
transclude X into B and Y into A, and then save, we have a REAL problem: 
we cannot save B before we know the global identity of Y, so we need to 
save A, but we cannot save A before we know the global identity of X...

Additionally, we do not know a new cell's global id before we save it, 
which is annoying.


There's a simple solution for this: Don't use block ids as part of the 
cell ids. Instead, use a secure random number. -- We generate one when 
Gzz starts up and then increment a counter each time we create a new 
cell, yielding 'FOO-nnn' cell ids as before. (However, we can encode 
them as base64, sparing us some more screen space.)

I know this slows down startup, because we need to initialize the random 
number generator. BUT, we need to do that anyway: without it, we cannot 
save Mediaserver blocks (we currently do, but it's not secure when 
publishing). So there's no additional cost in this really.

When the speed penalty gets too annoying (especially when running the 
tests), we can use a native implementation of SecureRandom that uses 
/dev/urandom (there's one around somewhere, someone should look into it).


- Benja