bugfix:server start and handle remain branch session may cause NPE

[Justice-love]

Ⅰ. Describe what this PR did

server start and handle remain branch session may cause it

Ⅱ. Does this pull request fix one issue?

1. BranchSession does not encode/decode branchType
2. DefaultCoordinator and SessionHolder init and start schedule handle rollback session
3. handle remain branch session ,such as rollback session, send BranchRollbackRequest
4. encode BranchRollbackRequest, append branchType code byteBuffer.put((byte)this.branchType.ordinal());
5. cause NPE and close channel

            future.addListener(new ChannelFutureListener() {
                @Override
                public void operationComplete(ChannelFuture future) {
                    if (!future.isSuccess()) {
                        MessageFuture messageFuture = futures.remove(rpcMessage.getId());
                        if (messageFuture != null) {
                            messageFuture.setResultMessage(future.cause());
                        }
                        destroyChannel(future.channel());
                    }
                }
            });

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-io]

Codecov Report

Merging #1237 into develop will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@              Coverage Diff              @@
##             develop    #1237      +/-   ##
=============================================
- Coverage      44.94%   44.92%   -0.03%     
  Complexity      1488     1488              
=============================================
  Files            253      253              
  Lines          10274    10279       +5     
  Branches        1335     1337       +2     
=============================================
  Hits            4618     4618              
- Misses          5070     5072       +2     
- Partials         586      589       +3

Impacted Files	Coverage Δ	Complexity Δ
...in/java/io/seata/server/session/BranchSession.java	78.19% <66.66%> (-0.72%)	41 <0> (ø)
...server/store/file/FileTransactionStoreManager.java	45.99% <0%> (-1.05%)	19% <0%> (ø)

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[Justice-love]

这个bug会导致RM注册上来的通道被关闭, 会有大量通道被关闭异常, 使用方非常难以判断真是问题

    @Override
    public Object sendSyncRequest(String resourceId, String clientId, Object message,
                                  long timeout) throws TimeoutException {
        Channel clientChannel = ChannelManager.getChannel(resourceId, clientId);
        if (clientChannel == null) {
            throw new RuntimeException("rm client is not connected. dbkey:" + resourceId
                + ",clientId:" + clientId);

        }
        return sendAsyncRequestWithResponse(null, clientChannel, message, timeout);
    }

        retryRollbacking.scheduleAtFixedRate(() -> {
            try {
                handleRetryRollbacking();
            } catch (Exception e) {
                LOGGER.info("Exception retry rollbacking ... ", e);
            }
        }, 0, rollbackingRetryDelay, TimeUnit.SECONDS);

[slievrly]

LGTM