Fix race condition for last_cd_tsc initialization
Fix race condition for last_cd_tsc initialization

The thread sanitizer identified the initialization of last_cd_tsc
to 0 as a race condition because every thread would set the value
to 0 on start. This commit changes things so the initialization is
only done by scheduler thread 0 (just as all other last_cd_tsc
manipulations).

kulibali/ponyc,sgebbie/ponyc,dipinhora/ponyc,dipinhora/ponyc,kulibali/ponyc,cquinn/ponyc,ponylang/ponyc,cquinn/ponyc,jemc/ponyc,kulibali/ponyc,sgebbie/ponyc,cquinn/ponyc,ponylang/ponyc,sgebbie/ponyc,dipinhora/ponyc,jemc/ponyc,ponylang/ponyc,kulibali/ponyc,jemc/ponyc,sgebbie/ponyc,cquinn/ponyc,sgebbie/ponyc