Optimize functions that use GetTime() multiple times by calling GetTime() just once (when possible/meaningful to do so)

[dsidirop]

Given that GetTime() reaches out all the way to the underlying OS it kinda makes sense to snapshot and reuse the timestamps it generates when possible. An example of before-after:

Before:

visibilityscan:SetScript("OnUpdate", function()
  if (this.limit or 1) > GetTime() then return end

  this.limit = GetTime() + .2
  [...]

After:

visibilityscan:SetScript("OnUpdate", function()
  local now = GetTime()
  if (this.limit or 1) > now then
    return
  end

  this.limit = now + .2
  [...]