Add --testnet mainnet and start HTTP server before genesis (#1862)
## Issue Addressed
NA
## Proposed Changes
- Adds support for `--testnet mainnet`
- Start HTTP server prior to genesis
## Additional Info
**Note: This is an incomplete work-in-progress. Use Lighthouse for mainnet at your own risk.**
With this PR, you can check the deposits:
```bash
lighthouse --testnet mainnet bn --http
```
```bash
curl localhost:5052/lighthouse/eth1/deposit_cache | jq
```
```json
{
"data": [
{
"deposit_data": {
"pubkey": "0x854980aa9bf2e84723e1fa6ef682e3537257984cc9cb1daea2ce6b268084b414f0bb43206e9fa6fd7a202357d6eb2b0d",
"withdrawal_credentials": "0x00cacf703c658b802d55baa2a5c1777500ef5051fc084330d2761bcb6ab6182b",
"amount": "32000000000",
"signature": "0xace226cdfd9da6b1d827c3a6ab93f91f53e8e090eb6ca5ee7c7c5fe3acc75558240ca9291684a2a7af5cac67f0558d1109cc95309f5cdf8c125185ec9dcd22635f900d791316924aed7c40cff2ffccdac0d44cf496853db678c8c53745b3545b"
},
"block_number": 3492981,
"index": 0,
"signature_is_valid": true
},
{
"deposit_data": {
"pubkey": "0x93da03a71bc4ed163c2f91c8a54ea3ba2461383dd615388fd494670f8ce571b46e698fc8d04b49e4a8ffe653f581806b",
"withdrawal_credentials": "0x006ebfbb7c8269a78018c8b810492979561d0404d74ba9c234650baa7524dcc4",
"amount": "32000000000",
"signature": "0x8d1f4a1683f798a76effcc6e2cdb8c3eed5a79123d201c5ecd4ab91f768a03c30885455b8a952aeec3c02110457f97ae0a60724187b6d4129d7c352f2e1ac19b4210daacd892fe4629ad3260ce2911dceae3890b04ed28267b2d8cb831f6a92d"
},
"block_number": 3493427,
"index": 1,
"signature_is_valid": true
},
```
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