HBASE-20856 PITA having to set WAL provider in two places (for branch-2)

[taklwu]

With this change if hbase.wal.meta_provider is not explicitly set,
it uses whatever set with hbase.wal.provider. this change avoids a use
case of unexpectedly using two different providers when only
hbase.wal.provider is set to non-default but not hbase.wal.meta_provider.

This change also include document (architecture.adoc) update

Also, this is a port from master to branch-2

[taklwu]

• mvn clean install -DskipTests

[INFO] Apache HBase - Testing Util ........................ SUCCESS [  5.418 s]
[INFO] Apache HBase - Thrift .............................. SUCCESS [  8.914 s]
[INFO] Apache HBase - RSGroup ............................. SUCCESS [  6.067 s]
[INFO] Apache HBase - Shell ............................... SUCCESS [  6.364 s]
[INFO] Apache HBase - Coprocessor Endpoint ................ SUCCESS [  6.921 s]
[INFO] Apache HBase - Integration Tests ................... SUCCESS [  7.626 s]
[INFO] Apache HBase - Rest ................................ SUCCESS [  6.733 s]
[INFO] Apache HBase - Examples ............................ SUCCESS [  5.762 s]
[INFO] Apache HBase - Shaded .............................. SUCCESS [  0.324 s]
[INFO] Apache HBase - Shaded - Client (with Hadoop bundled) SUCCESS [  2.206 s]
[INFO] Apache HBase - Shaded - Client ..................... SUCCESS [  2.067 s]
[INFO] Apache HBase - Shaded - MapReduce .................. SUCCESS [  3.839 s]
[INFO] Apache HBase - External Block Cache ................ SUCCESS [  3.338 s]
[INFO] Apache HBase - Assembly ............................ SUCCESS [ 11.708 s]
[INFO] Apache HBase Shaded Packaging Invariants ........... SUCCESS [  3.201 s]
[INFO] Apache HBase Shaded Packaging Invariants (with Hadoop bundled) SUCCESS [  1.641 s]
[INFO] Apache HBase - Archetypes .......................... SUCCESS [  0.070 s]
[INFO] Apache HBase - Exemplar for hbase-client archetype . SUCCESS [  4.236 s]
[INFO] Apache HBase - Exemplar for hbase-shaded-client archetype SUCCESS [  4.296 s]
[INFO] Apache HBase - Archetype builder ................... SUCCESS [  0.659 s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 04:37 min
[INFO] Finished at: 2018-08-01T12:59:47-07:00
[INFO] Final Memory: 287M/1797M
[INFO] ------------------------------------------------------------------------

• mvn clean test -Dtest=TestShutdownWhileWALBroken,TestAsyncLogRolling,TestLogRolling,TestWALFactory

[INFO] -------------------------------------------------------
[INFO]  T E S T S
[INFO] -------------------------------------------------------
[INFO] Running org.apache.hadoop.hbase.regionserver.TestShutdownWhileWALBroken
[INFO] Tests run: 2, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 41.829 s - in org.apache.hadoop.hbase.regionserver.TestShutdownWhileWALBroken
[INFO] Running org.apache.hadoop.hbase.regionserver.wal.TestAsyncLogRolling
[INFO] Tests run: 4, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 146.535 s - in org.apache.hadoop.hbase.regionserver.wal.TestAsyncLogRolling
[INFO] Running org.apache.hadoop.hbase.regionserver.wal.TestLogRolling
[INFO] Tests run: 5, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 235.697 s - in org.apache.hadoop.hbase.regionserver.wal.TestLogRolling
[INFO] Running org.apache.hadoop.hbase.wal.TestWALFactory
[INFO] Tests run: 11, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 28.999 s - in org.apache.hadoop.hbase.wal.TestWALFactory
[INFO] 
[INFO] Results:
[INFO] 
[INFO] Tests run: 22, Failures: 0, Errors: 0, Skipped: 0
[INFO] 
[INFO] 
[INFO] --- maven-surefire-plugin:2.21.0:test (secondPartTestsExecution) @ hbase-server ---
[INFO] Tests are skipped.
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 08:52 min
[INFO] Finished at: 2018-08-01T14:13:32-07:00
[INFO] Final Memory: 106M/1711M
[INFO] ------------------------------------------------------------------------

[taklwu]

merged as 690d29b