[Serum] Populate `seqNum` on `Event` and implement `loadFillsSince`

[lidatong]

Thank you for implementing this client library for Serum! I really appreciate the open-source efforts to make the Dex accessible, and I'd like to make my own contribution.

Description

This PR makes two changes:

1. Populates the seqNum on the Event type in decodeEventQueue
2. Implements a new function loadFillsSince

Why

It appears based on git blame that seqNum was added after decodeEventQueue, and decodeEventQueue was never updated.

I think it would be useful to populate the seqNum given it is available as a field, and there isn't a way to uniquely determine whether you've processed a Fill without it.

I also implemented an additional loadFillsSince API function, since there is already functionality to decodeEventsSince, and I think it would useful to be able to loadFillsSince a given seqNum

Testing

I manually against the Serum Validator node: https://solana-api.projectserum.com

Goal was to test that the seqNum is calculated correctly, being careful to make sure it handles the u32overflow case.

Here is a sample test output. Notice that head seqNum is 26178226, count is 4, user-requested limit is 5.

Doing the logic and calculating the expected case by hand:

Header seqNum: 26178226 Count: 4 Ending seqNum: 26178226 + 4 - 1 = 26178229 Starting seqNum: endSeqNum - limit = 26178225

The test output below correctly indicates the modified decodeEventQueue returns 5 fill events with the seqNum starting from 26178225:

{
  head: 967,
  count: 4,
  seqNum: 26178226
}
// decodeEvents: requested limit 5

[
  {
    orderId: <BN: 6c0d1fffffffffefbe0a9>,
    seqNum: 26178225
  },
  {
    orderId: <BN: 5b706fffffffffefbe0af>,
    seqNum: 26178226
  },
  {
    orderId: <BN: 4f414fffffffffefbe0b0>,
    seqNum: 26178227
  },
  {
    orderId: <BN: 630defffffffffefbe0b3>,
    seqNum: 26178228
  },
  {
    orderId: <BN: 82fc00000000001041f4a>,
    seqNum: 26178229
  }
]