Bump async transport _WAIT_PERIOD to 60s (census-instrumentation#354)

Fixes census-instrumentation#353.

Author: c24t

opencensus/common/transports/async.py

@@ -24,7 +24,7 @@
 
 _DEFAULT_GRACE_PERIOD = 5.0  # Seconds
 _DEFAULT_MAX_BATCH_SIZE = 10
-_WAIT_PERIOD = 1.0  # Seconds
+_WAIT_PERIOD = 60.0  # Seconds
 _WORKER_THREAD_NAME = 'opencensus.common.Worker'
 _WORKER_TERMINATOR = object()