tip selection: set default alpha value to 0.001

[alon-e]

Description

This PR sets the default alpha value for the random walk to 0.001,
in order for slower but honest users to still have a good chance of getting their transactions confirmed.

This value is based on both:

1. an analytical calculation of the probability of a transaction with slower PoW time H ~= 5mins to still get confirmed (assuming a constant lambda = 10 TPS and H >> h).

2. validation (of calculation) by running a private testnet (w/ 12TPS spam) where slower transactions were issued and confirmation rates were measured.

Fixes #801

Type of change

• Parameter tuning

How Has This Been Tested?

see Description

Checklist:

Please delete items that are not relevant.

• My code follows the style guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes