Members key share retrieval, decryption and submission

[jumaffre]

Addresses 5. and most of 6. and 6'. in #51

This PR adds two new member RPC getEncryptedRecoveryShare and submitRecoveryShare for members to easily retrieve their encrypted shares and submit the then-decrypted share.

To achieve this, cchost now outputs the network public encryption key on startup for members to easily decrypt their shares (using cryptobox). This was slightly painful since mbedtls does not yet support x25519 keys and we have to construct the PEM public key manually (see 25519.h).

For now, decrypted shares are stored in the node's memory on submitRecoveryShare and once the number of submit shares matches the threshold (for now, the number of active members), CCF combines the shares, reconstruct k_z and decrypt the ledger secrets in ccf.shares.

There is a new variant of the recovery_test e2e test to test the retrieval, decryption (using pynacl) and submission of shares.

Next:

• Plug the share combination into our existing recovery scheme (i.e. members can only submit their shares once the recovery proposal is passed and once the shares have been assembled, the recovery of the private ledger can start).

[codecov-io]

Codecov Report

Merging #932 into master will decrease coverage by 0.42%.
The diff coverage is 53.01%.

@@            Coverage Diff             @@
##           master     #932      +/-   ##
==========================================
- Coverage   68.96%   68.54%   -0.42%     
==========================================
  Files         100      101       +1     
  Lines        7802     7939     +137     
==========================================
+ Hits         5380     5441      +61     
- Misses       2422     2498      +76

Flag	Coverage Δ
#unit_BFT	68.54% <53.01%> (-0.41%)	⬇️
#unit_CFT	68.54% <53.01%> (-0.42%)	⬇️

Impacted Files	Coverage Δ
src/node/values.h	80% <ø> (ø)	⬆️
src/node/networkstate.h	100% <ø> (ø)	⬆️
src/node/ledgersecrets.h	14.67% <0%> (-0.4%)	⬇️
src/node/genesisgen.h	48.06% <0%> (-2.76%)	⬇️
src/node/rpc/nodecalltypes.h	100% <100%> (ø)	⬆️
src/node/secretshare.h	81.82% <100%> (ø)	⬆️
src/node/rpc/nodefrontend.h	77.25% <100%> (ø)	⬆️
src/node/rpc/memberfrontend.h	61.92% <18.46%> (-6.21%)	⬇️
src/node/networkencryption.h	28.57% <28.57%> (ø)
src/tls/25519.h	84.62% <84.62%> (+7.69%)	⬆️
... and 2 more

[ghost]

retrieve_encrypted_share@5875 aka 20200310.25 vs master ewma over 30 builds from 5516 to 5855

[jumaffre]

I'll merge this PR now as all the comments have been addressed. The key shares feature is very much work in progress and I expect that most of the code written so far will be reworked, or at least refactored.