The new module lib.tsc provides a generic Time Stamp Counter mechanism
for measuring time intervals based on different time sources. One
such source makes use of the CPU's TSC register via the rdtsc
instruction for low-latency timing purposes.
The rdtsc method is made the default time source for the token bucket,
formerly provided by core.lib, now moved to a separate module
lib.token_bucket. The commit also includes documentation for the token
bucket, which was previously missing.