Galera MTR Tests: Instruct xtrabackup SST's socat to use 1024-bit DH keys to avoid 'error:14082174:SSL routines:SSL3_CHECK_CERT_AND_ALGORITHM:dh key too small'

Galera MTR Tests: Tests for GRA*.log files, replication bundle, preordered events, forced binlog format

Galera MTR Tests: An end-to-end test with restoring a node from xtrabackup; a test for restoring the primary component via pc.bootstrap

Galera MTR Tests: mark all tests operating on large data sets with --source include/big-test.inc to help with Valgrind

Galera MTR Tests: Use SET GLOBAL when setting wsrep_replicate_myisam, as it is a GLOBAL variable in MySQL Galera Cluster and SESSION in Percona XTraDB Cluster

Before this, FLUSH TABLES and FLUSH TABLES t1 (REFRESH_TABLES hereafter) were executed locally,
which also had implications for GTID. MySQL async replication does
replicate FLUSH TABLES.

Hence, this commit

a)  Adds REFRESH_TABLES to TOI.

b) Skips LOCK-based REFRESH_TABLES from TOI, ie. FLUSH TABLES WITH READ
LOCK and FLUSH TABLES FOR EXPORT.

c) Executes them after reload_acl_and_cache. This is done since
reload_acl_and_cache checks for thd->locked_tables_mode and does upgrade
of MDL locks accordingly. A failure here means REFRESH_TABLES cannot
run.

    i) This is also important since LOCK TABLES is not replicated in
    galera.

d) Test galera_flush has been modified for

    i) wsrep_replicate_myisam SESSION variable.

    ii) Testing REFRESH_TABLES.

    iii) Testing REFRESH_TABLES + LOCK TABLES.

e) Use different WSREP_TO_ISOLATION_BEGIN for global FLUSH TABLES and FLUSH TABLES <table,>.

  - errno handling in wsp::env::append() method, where error could be returned by mistake
  - return code of sst_prepare_other() when pthread_create() fails - it was returning positive error code which by convention is treated as success.

  1. Passes wsrep_sst_auth_value to SST scripts via WSREP_SST_OPT_AUTH envronmental variable, so it never appears on the command line
  2. In mysqldump and xtrabackup* SST scripts which rely on MySQL authentication, instead of passing password on the command line, SST script sets MYSQL_PWD environment variable, so that password also never appears on the mysqldump/innobackupex command line.

- fix for THD::m_digest initialization, according to Raghu

- changed BF thread's MDL wait to never timeout
- all explicit locks are now honored by BF threads

sessions to cover all exclusive MDL locks