new UCI option "Time Buffer"
expose the hardcoded 60msec time buffer (for GUI lag) to the user. In some
use cases, we need an extremely small value (eg. cutechess-cli testing in
1+0.01). In other use cases, we want a large value (eg. Arena which is the
slowest thing in the world, and constaltly flags engines incorrectly due
to its own lag or incorrect time measurement).

Also make the default value a bit more conservative (100 msec). And
generalize use of int64_t for times measures in msec everywhere (internally
was int64_t, but at UCI level was int for some reason).