Robotics Service Bus

Allow out-of-order updates of cells in formatting/timeline.lisp

fixes #1779

Previously, newly added cells would be updated once when processing a
batch of events and then never again leading to dropping of events (or
batches) which were processed out-of-order (w.r.t. to the configured
timestamp). Now, cells can be updated after their initial population.

• formatting/timeline.lisp (%cell): added slots count and max-size
  (cell-glyph): new function; return cached glyph or compute glyph
  (%cell-update): new function; update count and max-size slots and
  clear cached glyph
  (glyph-for-data): new function; based on `glyph-for-events'
  (timeline::events): removed initarg; renamed accessor style-{events ->
  %events}; mention sorting requirement
  (format-event eql :trigger timeline t): use `cell-glyph'; do not pop
  off first cache element for update since it is now mutable
  (format-event :around t timeline t): use `merge' to sort queued events
  (fill-cache!): re-visit cache cells if necessary; removed much of the
  intricate update logic which became unnecessary
  (glyph-for-events): removed; replaced by `glyph-for-data'

Timeline shows one second of future time in formatting/timeline.lisp

refs #1779

With the previous upper bound of "now", events could be dropped when the
timing was slightly off.

• formatting/timeline.lisp (timeline): added default initarg upper-bound
  to set upper bound one second into the future
• formatting/event-style-timeline.lisp (basic-timeline-style): likewise;
  is needed to be passed to sub-styles