soroban-rpc: Add writethrough cache for Config ledger entries

[2opremio]

After #692 I noticed that the DB access to config ledger entries is a big part of the preflight computation.

This PR adds a writethrough cache for config ledger entries.

Before the cache:

goos: darwin
goarch: arm64
pkg: github.com/stellar/soroban-tools/cmd/soroban-rpc/internal/preflight
BenchmarkGetPreflight
BenchmarkGetPreflight/In-memory_storage
BenchmarkGetPreflight/In-memory_storage-12         	    6112	    193728 ns/op
BenchmarkGetPreflight/DB_storage
BenchmarkGetPreflight/DB_storage-12                	    2766	    430002 ns/op
PASS

After adding the cache:

goos: darwin
goarch: arm64
pkg: github.com/stellar/soroban-tools/cmd/soroban-rpc/internal/preflight
BenchmarkGetPreflight
BenchmarkGetPreflight/In-memory_storage
BenchmarkGetPreflight/In-memory_storage-12         	    6169	    192616 ns/op
BenchmarkGetPreflight/DB_storage
BenchmarkGetPreflight/DB_storage-12                	    3717	    324078 ns/op
PASS

I am not sure it's worth the extra complexity though, considering it brings us down from 0.43 ms to 0.32 ms per (simple) preflight.

[tamirms]

@2opremio doesn't the cache keep growing indefinitely? I would be worried about the memory consumption because I think the cache would just keep growing as long as soroban rpc is up. The other concern is that the cache is copied whenever a new read transaction is created. So if the cache is already large lots of concurrent requests could spike memory usage even more.

[2opremio]

doesn't the cache keep growing indefinitely?

It doesn't, because we only store the config ledger entries

The other concern is that the cache is copied whenever a new read transaction is created

Valid concern, but the number of possible ledger entries is small (~15)

[2opremio]

@tamirms I feel likeI didn't supply enough context. We only cache config ledger entries. The preflight computation needs config parameters from the network and it fetches them (around 10 ledger entries) every time it runs.

The number of config ledger entries is limited (around 15 of them, see https://github.com/stellar/stellar-xdr/blob/next/Stellar-contract-config-setting.x#L200-L217 ) so we don't have to worry about eviction.

[2opremio]

@tamirms PTAL