MEDIUM: log: change a few log tokens to make them easier to remember

MEDIUM: log: change a few log tokens to make them easier to remember

Some log tokens have evolved in a way that is not completely logical.
For example, frontend tokens sometimes begin with an 'f' and sometimes
with an 'F'. Same for backend and server.

So let's change a few cases without disrupting compatibility with existing
setups :

  Bi => bi
  Bp => bp
  Ci => ci
  Cp => cp
  Fi => fi
  Fp => fp
  Si => si
  Sp => sp
  cc => CC
  cs => CS
  st => ST

The old ones are still supported but deprecated and will be unsupported by
the 1.5 release. However, a warning message is emitted when they're encounterd
and it indicates what token should be used to replace them.