Implement LTTNG_UST_BLOCKING_RETRY_TIMEOUT

Implement LTTNG_UST_BLOCKING_RETRY_TIMEOUT

Add LTTNG_UST_BLOCKING_RETRY_TIMEOUT environment variable:

       LTTNG_UST_BLOCKING_RETRY_TIMEOUT
           Maximum duration (milliseconds) to retry event tracing when
           there’s no space left for the event record in the
           sub-buffer.

           0 (default)
               Never block the application.

           Positive value
               Block the application for the specified number of
               milliseconds. If there’s no space left after this
               duration, discard the event record.

           Negative value
               Block the application until there’s space left for the
               event record.

           This option can be useful in workloads generating very
           large trace data throughput, where blocking the application
           is an acceptable trade-off to prevent discarding event
           records.

               Warning
               Setting this environment variable to a non-zero value
               may significantly affect application timings.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: "Carlos O'Donell" <carlos@redhat.com>