Change directIO configuration names to avoid confusing

[hangc0276]

Motivation

We have the following configurations from the DirectIO entry logger.

dbStorage_directIOEntryLoggerTotalWriteBufferSizeMb
dbStorage_directIOEntryLoggerTotalReadBufferSizeMb
dbStorage_directIOEntryLoggerReadBufferSizeMb

In fact, the units are bytes instead of bits. But from the first sight of the configuration name, users may be confused by the units, because we use Mb instead of MB.
I suggest changing the configuration name suffix from Mb to MB to avoid confusing.

Modifications

• Change dbStorage_directIOEntryLoggerTotalWriteBufferSizeMb to dbStorage_directIOEntryLoggerTotalWriteBufferSizeMB
• Change dbStorage_directIOEntryLoggerTotalReadBufferSizeMb to dbStorage_directIOEntryLoggerTotalReadBufferSizeMB
• Change dbStorage_directIOEntryLoggerReadBufferSizeMb to dbStorage_directIOEntryLoggerReadBufferSizeMB
• Add documents in conf/bk_server.conf

[codecov-commenter]

Codecov Report

Merging #3791 (eb17704) into master (13ccc4c) will decrease coverage by 7.85%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #3791      +/-   ##
============================================
- Coverage     68.16%   60.31%   -7.85%     
+ Complexity     6719     5816     -903     
============================================
  Files           473      473              
  Lines         40952    40952              
  Branches       5236     5236              
============================================
- Hits          27913    24700    -3213     
- Misses        10778    14033    +3255     
+ Partials       2261     2219      -42     

Flag	Coverage Δ
bookie	39.71% <ø> (+0.02%)	⬆️
client	?
remaining	29.67% <ø> (-0.02%)	⬇️
replication	41.45% <ø> (+0.14%)	⬆️
tls	21.00% <ø> (-0.10%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...bookkeeper/bookie/storage/ldb/DbLedgerStorage.java	74.61% <ø> (-5.08%)	⬇️
...a/org/apache/bookkeeper/client/api/BookKeeper.java	0.00% <0.00%> (-100.00%)	⬇️
.../bookkeeper/proto/checksum/DummyDigestManager.java	0.00% <0.00%> (-100.00%)	⬇️
...okkeeper/client/BookieAddressResolverDisabled.java	0.00% <0.00%> (-100.00%)	⬇️
...keeper/proto/ReadLastConfirmedAndEntryContext.java	0.00% <0.00%> (-100.00%)	⬇️
...kkeeper/client/impl/LastConfirmedAndEntryImpl.java	0.00% <0.00%> (-95.84%)	⬇️
...per/client/DynamicWeightedRandomSelectionImpl.java	0.00% <0.00%> (-92.07%)	⬇️
...kkeeper/client/DefaultEnsemblePlacementPolicy.java	0.00% <0.00%> (-87.81%)	⬇️
...per/client/RegionAwareEnsemblePlacementPolicy.java	0.00% <0.00%> (-81.70%)	⬇️
...a/org/apache/bookkeeper/client/UpdateLedgerOp.java	0.00% <0.00%> (-78.27%)	⬇️
... and 127 more

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