Add _approx_count_distinct as a builtin reduce function (#1346)

This introduces a new builtin reduce function, which uses a HyperLogLog
algorithm to estimate the number of distinct keys in the view index. The
precision is currently fixed to 2^11 observables andtherefore uses
approximately 1.5 KB of memory.

It also introduces a finalize step which can be used to improve the
efficiency of other builtin reduce functions going forward.

Closes COUCHDB-2971

1 files changed, 24 insertions, 0 deletions

diff --git a/LICENSE b/LICENSE
index a209352a0..6034c717e 100644
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+For the src/hyper component:
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