Introduce LTTNG_UST_ALLOW_BLOCKING env. var.

Introduce the LTTNG_UST_ALLOW_BLOCKING env. var. to control whether
applications are allowed to block when a buffer is full. If set, it
allows the tracer to block the application when buffers are full.

The blocking is now controlled by a per-channel configuration option in
the LTTng control interface for channels with the "--blocking-timeout"
parameter, which is specified in usec (or -1 to block forever).

This replaces the LTTNG_UST_BLOCKING_RETRY_TIMEOUT env. var., which
actually never made it into a stable release (we therefore remove this
env. var).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>