Preallocate addresses in GetAddr based on nNodes

> make && ./src/bench/bench_bitcoin --filter=AddrManGetAddr --min-time=1000

Before:
```
|               ns/op |                op/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|           76,852.79 |           13,011.89 |    0.4% |      1.07 | `AddrManGetAddr`
|           76,598.21 |           13,055.14 |    0.2% |      1.07 | `AddrManGetAddr`
|           76,296.32 |           13,106.79 |    0.1% |      1.07 | `AddrManGetAddr`
```
After:
```
|               ns/op |                op/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|           65,966.97 |           15,159.10 |    0.3% |      1.07 | `AddrManGetAddr`
|           66,075.40 |           15,134.23 |    0.2% |      1.06 | `AddrManGetAddr`
|           66,306.34 |           15,081.51 |    0.3% |      1.06 | `AddrManGetAddr`
```

src/addrman.cpp

@@ -818,6 +818,7 @@ std::vector<CAddress> AddrManImpl::GetAddr_(size_t max_addresses, size_t max_pct
     // gather a list of random nodes, skipping those of low quality
     const auto now{Now<NodeSeconds>()};
     std::vector<CAddress> addresses;
+    addresses.reserve(nNodes);
     for (unsigned int n = 0; n < vRandom.size(); n++) {
         if (addresses.size() >= nNodes)
             break;