Fix hanging during SSL_write for large packets

[old-bear]

What problem does this PR solve?

Issue Number:

Problem Summary:
When sending large packet under SSL, brpc could stuck as data still resides in BIO buffer.

What is changed and the side effects?

Changed: Disable BIO for SSL for 2 reasons:

1. The current implementation makes it very hard to handle EAGAIN from both SSL_write and BIO_flush.
2. BIO is optional and kind of outdate for modern TCP, which already has buffer mechanism.

Side effects:

• Performance effects: Should be no impact

• Breaking backward compatibility: No

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[wwbmmm]

LGTM

[chenBright]

Is BIO_flush still needed?

brpc/src/butil/iobuf.cpp

Lines 934 to 939 in 7229c36

	if (BIO_wpending(wbio) > 0) {
	int rc = BIO_flush(wbio);
	if (rc <= 0 && BIO_fd_non_fatal_error(errno) == 0) {
	// Fatal error during BIO_flush
	*ssl_error = SSL_ERROR_SYSCALL;
	return rc;

[old-bear]

Is BIO_flush still needed?

brpc/src/butil/iobuf.cpp

Lines 934 to 939 in 7229c36

	if (BIO_wpending(wbio) > 0) {
	int rc = BIO_flush(wbio);
	if (rc <= 0 && BIO_fd_non_fatal_error(errno) == 0) {
	// Fatal error during BIO_flush
	*ssl_error = SSL_ERROR_SYSCALL;
	return rc;

No. BIO_wpending will return 0, so this code will not be reached. For clarity, I have commented out the code and left a note.