The exporters specification states that two built-in span processors should be
implemented, the simple processor span and the batch processor span.

This commit implements the later, it is mainly based on the opentelemetry/java
one.

The algorithm implements the following logic:
- a condition variable is used to notify the worker thread in case the queue
is half full, so that exporting can start before the queue gets full and spans
are dropped.
- export is called each schedule_delay_millis if there is a least one new span
to export.
- when the processor is shutdown all remaining spans are exported.

- use // instead of /
- set daemon=true
- fix error message for max_batch_size
- change schedule_delay_millis' type to float
- make flush internal method

it doesn't work sometimes when using the exact same value of schedule_delay_millis,
add some room

- log when dropping spans
- check if processor is already shutdown
- avoid using RLock
- prereserve span array
- introduce variable to avoid sending notification storm

- export returns nothing
- added another test that sends more than max_queue_size elements
- removed notified extra variable
- move preallocated list to class