This PR introduces as sharded mutex within the ChainIndex#GetTipsetByHeight.
It also replaces a go map with xsync.Map which doesn't require locking.
The lock is taken when it appears that ChainIndex filling work should be
started. After claiming the lock, the status of the cache is rechecked,
if the entry is still missing, the fillCache is started.
Thanks to @snissn and @arajasek for debugging and taking initial stabs at this.
Supersedes #10866 and 10885
Signed-off-by: Jakub Sztandera <kubuxu@protocol.ai>
