Introduce separate stats for indexbuild

Adds extra stats (hit/miss) for the 'indexbuild'
Reverts the XCRemoteCache circuit breaker when ACTION=indexbuild. Once #9 is merged, XCRemoteCache can reuse artifacts for the indexbuild too.

Sample output when building and waiting a bit to finish all indexbuild steps :

$ xcprepare stats
hit_count: 4004
miss_count: 44
local_cache_bytes: 7423500288
indexing_hit_count: 672
indexing_miss_count: 10

Note: This change is backward and forward compatible.

spotify / XCRemoteCache