Create SiaDirHealth stuct for health updates and bubble up numStuckChunks

MERGE REQUEST

Changes to Work Flow

With the updates coming from the health loop and the repair loop it became obvious that there was a number of siadir metadata fields relating to the health of the siadir that were getting passed around. I created an exported SiaDirHealth struct to make it a little cleaner to update and pass around this information. I added numStuckChunks as a field that is bubbled up through BubbleHealth.

Additionally I expanded the siafile Health method to calculate health and stuckHealth.

Pending !3374 (merged)

Closes #3400 (closed)

Testing Updates

• No new tests were added related to SiaDirHealth or numStuckChunks, just a helper method and expanding existing tests.
• Expanded TestBubbleHealth to probe case when file is stuck, this Closes #3377 (closed)
• Expanded TestFileHealth to probe health vs stuckHealth

Modules and Packages Impacted

• modules/renter
• modules/renter/siadir
• modules/renter/siafile