Use gamma distribution to pick fake outputs

[dternyak]

Referencing monero-project/monero#3528 (comment) and monero-project/monero#3528 (comment)

Based on findings in "An Empirical Analysis of Linkability in the Monero
Blockchain", the average time real outputs are spent in real-world conditions are 1.8 days. Thus, a truly random output selection of the entire history of the chain (as implemented today), allows for some (undetermined) statistical analysis in determining which outputs are likely to be decoys.

• Use a gamma distribution to pick fake outputs instead of selecting random outputs