tipSetCache.best() returns nil tipset

[hannahhoward]

Describe the bug
We've had various reports of panics due to the checkFunc in the eventsAPI.Called interface being passed a nil tipset (#2865, also from DealBot operators). We've resolved this on an individual basis by adding nil checks:
#3553
#3011

It seems like this will be an ongoing issue unless:

• We decide passing a nil tipset check to checkFunc is expected behavior, in which case, we should document it.
• We investigate and determine why tipsetCache.best() is returning nil tipsets (I have various hypothesis)

[hannahhoward]

Possible hypothesis and possible solution:

Why it happens:

• tipsets get reverted?
• thread sync/race conditions
• kind of hard to track down

Possible solutions:

Simple:
if tsc.cache[tsc.best] is nil, just fall back to loading directly from the API (i.e. tipsetCache already has a tipsetByHeight function (actually api.ChainGetTipsetByHeight when initialized in Events API -- if we added a chainHead function we could call that and supply it api.ChainHead in events API)

More complex:
Try to figure out how we're ending up in the condition where there is no value at tsc.best and resolve... probably not worth it.