optimize: eliminate the possibility of allocating too much direct memory

[ggndnn]

Ⅰ. Describe what this PR did

In FileTransactionStoreManager, sometimes, data buffer size was not under control. For example when data size to write was bigger than writerBuffer:

...
ByteBuffer byteBuffer = null;

if (bs.length + 4 > MAX_WRITE_BUFFER_SIZE) {
    //allocateNew
    byteBuffer = ByteBuffer.allocateDirect(bs.length + 4);
} else {
...

Or in function findTimeoutAndSave:

...
ByteBuffer byteBuffer = ByteBuffer.allocateDirect(totalSize);
for (byte[] bytes : listBytes) {
    byteBuffer.putInt(bytes.length);
    byteBuffer.put(bytes);
}
...

Those buffers' size was out of control, unlimited, that may be a hidden danger.

This PR makes fully use of the writerBuffer, eliminate the possibility of allocating too much memory.

Ⅱ. Does this pull request fix one issue?

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

Did.

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-io]

Codecov Report

Merging #1657 into develop will increase coverage by 0.17%.
The diff coverage is 50%.

@@              Coverage Diff              @@
##             develop    #1657      +/-   ##
=============================================
+ Coverage      48.17%   48.35%   +0.17%     
- Complexity      1846     1858      +12     
=============================================
  Files            365      365              
  Lines          13308    13325      +17     
  Branches        1645     1652       +7     
=============================================
+ Hits            6411     6443      +32     
+ Misses          6233     6213      -20     
- Partials         664      669       +5

Impacted Files	Coverage Δ	Complexity Δ
...server/store/file/FileTransactionStoreManager.java	52.63% <50%> (+6.63%)	27 <4> (+8)	⬆️
...in/java/io/seata/server/session/GlobalSession.java	85.02% <0%> (+0.48%)	68% <0%> (+1%)	⬆️
...in/java/io/seata/server/session/BranchSession.java	80.45% <0%> (+2.25%)	44% <0%> (+3%)	⬆️

---

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[slievrly]

LGTM, good job.

[jsbxyyx]

LGTM