Block breaking delay resets when not mining a block

Normally, there is a 6 game tick (0.3 second) delay between breaking blocks when not instant mining, however this delay is removed as soon as the player stops mining blocks, whether by releasing the Attack / Destroy button or by no longer looking at any blocks.

Steps to reproduce:
* Start mining a large area of blocks; the issue is most noticeable when the block is destroyed in 2 game ticks, such as in the case of netherrack when broken by an unenchanted diamond pickaxe.
* Repeatedly press the Attack / Destroy button at a rate of roughly 5 presses a second (for blocks that take longer to mine the button will need to be pressed slower), making sure the button is held down long enough on each press to destroy the block.

Expected result:
The blocks would be destroyed no faster than if the button was held down (in the case of a block broken in 2 ticks, 2.5 blocks broken per second)

Actual result:
The blocks are destroyed noticeably faster than holding the button, up to a theoretical maximum of over 6 blocks mined per second