Fix composite-aggs scan, by retrieving first batch.

[leonardbinet]

Fixes #101

[codecov-commenter]

Codecov Report

Merging #105 (f09878d) into master (39e50d9) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #105   +/-   ##
=======================================
  Coverage   90.04%   90.04%           
=======================================
  Files          45       45           
  Lines        2862     2862           
=======================================
  Hits         2577     2577           
  Misses        285      285           

Impacted Files	Coverage Δ
pandagg/index.py	80.84% <ø> (ø)
pandagg/node/mappings/abstract.py	92.30% <ø> (ø)
pandagg/tree/mappings.py	85.31% <ø> (ø)
pandagg/search.py	86.29% <100.00%> (ø)

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