As discussed in #966, this PR makes the size of the ring buffer used to send
data to userspace configurable. It changes the Python, Lua and C++ APIs to
expose this knob.

It also defaults the buffer size to a larger value (64 pages per CPU, an 8x
increase) for several tools which produce a lot of output, as well as making it
configurable in `trace` via a `-b` flag.

* Filter by process ID in ex4slower

* Updated the rest of the tools to filter by process ID

Convert some of the examples and tools to use the new helpers.
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>