Potential refactoring: proper use of context in readbuffer

[moio]

reabuffer.go code seems prone to leaks, race conditions and similar bugs because of its use of Cond (which is generally discouraged) and the lack of "typical" use of contexts.

Although I am not aware of any new specific bug, I believe a refactoring of this part could be beneficial for future readers given the crucial importance of this piece of architecture to the whole Rancher product family.

cc @aruiz14