Fix: split UST per UID/PID default values

The default per UID subbuffer size are changed from 4096 bytes to 131072
bytes (128k). The rest have the same values as before.

Some DEFAULT_CHANNEL_* macros are prefixed with an underscore making
sure they are not used elsewhere in the code base and only domain
specific values are used.

Fixes #524

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>