soroban-rpc: Add transactions to memory store

[2opremio]

What

Store transactions in the in-memory store. The key change is:

 // MemoryStore is an in-memory store of soroban events.
@@ -46,7 +30,8 @@ type MemoryStore struct {
        // all events occurring within a specific ledger.
        buckets []bucket
        // start is the index of the head in the circular buffer.
-       start uint32
+       start        uint32
+       transactions map[xdr.Hash]Transaction
 }

@@ -15,22 +12,9 @@ type bucket struct {
        ledgerSeq            uint32
        ledgerCloseTimestamp int64
        events               []event
+       // transactions in the memory store belonging to the ledger in the current bucket
+       // this is used for garbage-collecting the global memory store transactions when the bucket is evicted
+       transactionHashes []xdr.Hash
 }

There is also a lot of shuffling and renaming to account for the store not being used exclusively for events.

Why

#409

Known limitations

The store is not used yet (that will happen in an upcoming PR)

[tamirms]

I think it would be better to have two separate MemoryStore structs (one for transactions and one for events) instead of combining both transactions and events in the same struct.

Because we won't need to join events with transactions since we agreed not to include any events in the getTransaction() endpoint, I don't think it's necessary to have both events and transactions in the same data structure. Also we probably want to configure a different retention window for transactions since they're going to be much more short lived compared to events.

[2opremio]

I will try to use generics. I think it should be feasible.

[2opremio]

@tamirms I implemented your suggestion at #427

[tamirms]

@2opremio I like the implementation in #427 and I prefer it to the approach in this PR.