Embed fingerprints in TSDB

[owen-d]

Opening/closing this PR for visibility.
@cyriltovena and I had guessed that embedding the label hash (fingerprint) into TSDB would be a net gain for read speeds, but after implementing it, that does not seem the case:

benchmarks:

name                                         old time/op    new time/op    delta
Query_PostingsForMatchers/match_ns-4           4.19µs ± 0%    4.08µs ± 0%   ~     (p=1.000 n=1+1)
Query_PostingsForMatchers/match_ns_regexp-4    4.16µs ± 0%    4.07µs ± 0%   ~     (p=1.000 n=1+1)
Query_GetChunkRefs/match_ns-4                  22.5ms ± 0%    22.1ms ± 0%   ~     (p=1.000 n=1+1)
Query_GetChunkRefs/match_ns_regexp-4           22.2ms ± 0%    22.2ms ± 0%   ~     (p=1.000 n=1+1)

name                                         old alloc/op   new alloc/op   delta
Query_PostingsForMatchers/match_ns-4            80.0B ± 0%     80.0B ± 0%   ~     (all equal)
Query_PostingsForMatchers/match_ns_regexp-4     80.0B ± 0%     80.0B ± 0%   ~     (all equal)
Query_GetChunkRefs/match_ns-4                  33.9MB ± 0%    33.9MB ± 0%   ~     (p=1.000 n=1+1)
Query_GetChunkRefs/match_ns_regexp-4           33.9MB ± 0%    33.9MB ± 0%   ~     (p=1.000 n=1+1)

name                                         old allocs/op  new allocs/op  delta
Query_PostingsForMatchers/match_ns-4             4.00 ± 0%      4.00 ± 0%   ~     (all equal)
Query_PostingsForMatchers/match_ns_regexp-4      4.00 ± 0%      4.00 ± 0%   ~     (all equal)
Query_GetChunkRefs/match_ns-4                   7.77k ± 0%     7.77k ± 0%   ~     (all equal)
Query_GetChunkRefs/match_ns_regexp-4            7.77k ± 0%     7.77k ± 0%   ~     (all equal)

sizing:

-rw-r--r-- 1 owen users 57M Mar  9 09:23 /tmp/loki-tsdb-a
-rw-r--r-- 1 owen users 58M Mar  9 09:24 /tmp/loki-tsdb-b

I'd guess this is because most of our time is spent on other operations which eclipse the potential benefit of this, such as growing slices in GetChunkRefs:

ref #5428