Use fixed resources for sending

[marcomorain]

Introduce 2 limits on the resources used.

Use an Executor with a thread pool of max size 1 for sending HTTP requests
to Sentry. This thread pool will use 0 threads at idle, and reduce the
number of threads in use down to 0 after a minute of no exceptions.

Use a fixed queue for events. A queue of up to 50 events will be queued
for sending to Sentry. If more than 50 events are queued, the new events
will be discarded.

These two changes will add a fixed ceiling to the amount of resources the
Sentry client will use on the device.

[marcomorain]

@joshdholtz this will reduce the amount of resources used by the client.

This moves processing off the async task thread pool so the Sentry client won't block the app.