logicalplan: refactor distribution, fix bug in binary push down

[MichaHoffmann]

If one arm of a binary expression is a distribution point but not the binary expression itself, we would not distribute the other arm - this commit fixes this.

To fix this we have to refactor the distribution algorithm to compute the distribution points first and then do the actual modification of the AST.

[MichaHoffmann]

$ go test ./engine/... -run=NONE -fuzz=FuzzDistributedEngineQuery -fuzztime=2m -v
=== RUN   FuzzDistributedEngineQuery
fuzz: elapsed: 0s, gathering baseline coverage: 0/2706 completed
fuzz: elapsed: 3s, gathering baseline coverage: 37/2706 completed
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fuzz: elapsed: 1m51s, gathering baseline coverage: 2703/2706 completed
fuzz: elapsed: 1m52s, gathering baseline coverage: 2706/2706 completed, now fuzzing with 20 workers
fuzz: elapsed: 1m54s, execs: 2799 (32/sec), new interesting: 3 (total: 2709)
fuzz: elapsed: 1m57s, execs: 2922 (41/sec), new interesting: 9 (total: 2715)
fuzz: elapsed: 2m0s, execs: 3043 (40/sec), new interesting: 14 (total: 2720)
fuzz: elapsed: 2m2s, execs: 3043 (0/sec), new interesting: 14 (total: 2720)
--- PASS: FuzzDistributedEngineQuery (122.35s)
=== NAME  
PASS
ok  	github.com/thanos-io/promql-engine/engine	122.374s