capi: buffer blocks in a separate thread

[JacekGlen]

In this PR a separate thread is started with a callable object BlockProvider. This constantly adds new blocks to the block_buffer. The underlying class BoundedBuffer is thread-safe and handles two cases:

• insert item waits if the buffer is full
• pop item waits if the buffer is empty

By offloading the block reading to a separate thread we gain ~4% of the block execution time.