Writing the genesis ledger takes a long time

[mrmr1993]

Generating a genesis ledger with 300k accounts takes about 20s, but writing it to disk takes > 15 minutes (on my machine). If we want to test larger ledger sizes, this will quickly become unacceptably slow.

Generating a test ledger can be tested by passing a config file to the daemon or runtime_genesis_ledger.exe with a config such as

 {
   "genesis": {
     "genesis_state_timestamp": "2021-06-03T00:00:00Z"
   },
   "proof": {
     "ledger_depth": 19
   },
   "ledger": {
     "name": "test-300k",
     "num_accounts": 300000,
     "accounts": [
       {
         "pk": "B62qmnkbvNpNvxJ9FkSkBy5W6VkquHbgN2MDHh1P8mRVX3FQ1eWtcxV",
         "balance": "6000000000.000000000",
         "delegate": "B62qmnkbvNpNvxJ9FkSkBy5W6VkquHbgN2MDHh1P8mRVX3FQ1eWtcxV",
         "sk": null,
         "timing": null,
         "_comment": "This is the demo key, you can find the private key in dockerfiles/auxiliary_entrypoints/01-run-demo.sh"
       }] }
 }

[mrmr1993]

Proposed solution: use a batched rocksdb update to write all of the data at once. We already have this capability exposed on the database ledger; we just need to hook it up here.