Add BLS Batch Verification

[mstraka100]

Description

During consensus, changes single BLS verification to batch BLS verification.

Other changes

Small refactor of BLS signing code.

Tested

CI integration tests

Backwards compatibility

The signatures that are being verified do not need to be changed.

[piersy]

Coverage from tests in ./e2e_test/... for ./consensus/istanbul/... at commit f25b3786677c7658134fa863585262963f5bbe96

coverage: 49.6% of statements across all listed packages

coverage:  61.3% of statements in consensus/istanbul
coverage:  42.9% of statements in consensus/istanbul/announce
coverage:  55.5% of statements in consensus/istanbul/backend
coverage:   0.0% of statements in consensus/istanbul/backend/backendtest
coverage:  24.3% of statements in consensus/istanbul/backend/internal/replica
coverage:  62.5% of statements in consensus/istanbul/core
coverage:  50.0% of statements in consensus/istanbul/db
coverage:   0.0% of statements in consensus/istanbul/proxy
coverage:  64.4% of statements in consensus/istanbul/uptime
coverage:  51.8% of statements in consensus/istanbul/validator
coverage:  79.2% of statements in consensus/istanbul/validator/random

CommentID: e255b5c87d

[palango]

Closing this as we don't plan to include this in the short term.