RPC: Add reserve member function to `UniValue` and use it in `blockToJSON` function
This PR is motivated by https://github.com/bitcoin/bitcoin/issues/30495#issuecomment-2444881418,
It adds a reserve member function to UniValue and applies it within the blockToJSON function to pre-allocate memory, minimizing reallocation's.
Slight performance increase in verbosity 1
On master:
| ns/op | op/s | err% | total | benchmark |
|---|---|---|---|---|
| 190,342 | 5,254 | 2.3% | 0.01 | BlockToJsonVerbose1 |
| 34,812,292 | 28.73 | 1.0% | 0.38 | BlockToJsonVerbose2 |
| 34,457,167 | 29.02 | 1.0% | 0.38 | BlockToJsonVerbose3 |
On this PR:
| ns/op | op/s | err% | total | benchmark |
|---|---|---|---|---|
| 172,278 | 5,805 | 0.7% | 0.01 | BlockToJsonVerbose1 |
| 33,720,584 | 29.66 | 0.4% | 0.37 | BlockToJsonVerbose2 |
| 33,884,417 | 29.51 | 1.2% | 0.38 | BlockToJsonVerbose3 |