Bump default kernel, and UST per-uid/per-pid buffer size

LTTng with current default buffer size often lead to discarded events,
which is not something we want as a first user impression.

The choice of default buffer size were made conservatively around 2010.
Since then, the memory available on typical systems has increased, and
so has the amount of instrumentation available. As an example, the
mid-2010 Macbook Pro had 2GB ram. The current 2017 Macbook specification
states 8GB ram, for a 4-fold installed memory size increase.

Increase the kernel tracer buffer size from:
4 x 256kB per core
to:
4 x 1MB per core

Increase the UST tracer per-uid buffer size from:
4 x 128kB per core
to
4 x 512kB per core

Increase the UST tracer per-pid buffer size from:
4 x 4kB per core
to
4 x 16kB per core

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>