Refactor|Log|libdeng2: Keep a pool of shared log entry argument instances

To avoid repeatedly allocating and freeing memory for log entry
arguments, the existing ones are now kept in a shared pool, where
their values will simply be reset to new ones when taken into use
in new log entries.

Note that deep copying of string arguments still occurs.
Cleanup|Server|Client: Log levels and domains (continued)

Converted remaining Con_Message() calls in the network domain to
use LOG.