Rename "time" -> "timestamp" (terminology)

Rename "time" -> "timestamp" (terminology)

This patch renames the "time" concept to "timestamp" throughout.

The term "timestamp" is better to describe what used to be named "time",
for example a packet's beginning and end timestamps, and an event
record's timestamp.

"Time" has many definitions, one of which is duration, whereas the
only definition of "timestamp" in Antidote 10 is:

> a printed or stamped mark indicating a time or date of an event

which is pretty accurate.

Which is also why the timestamp members are named `timestamp_begin`,
`timestamp_end`, and `timestamp` in CTF 1.8.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>