Improve the performance of block cache keys

Doing a pure random read test on data that's 100% block cache, I see that we are spending quite some time in getBlockCacheKey:

"IPC Server handler 19 on 62023" daemon prio=10 tid=0x00007fe0501ff800 nid=0x6c87 runnable [0x00007fe0577f6000]
java.lang.Thread.State: RUNNABLE
at java.util.Arrays.copyOf(Arrays.java:2882)
at java.lang.AbstractStringBuilder.expandCapacity(AbstractStringBuilder.java:100)
at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:390)
at java.lang.StringBuilder.append(StringBuilder.java:119)
at org.apache.hadoop.hbase.io.hfile.HFile.getBlockCacheKey(HFile.java:457)
at org.apache.hadoop.hbase.io.hfile.HFileReaderV2.readBlock(HFileReaderV2.java:249)
at org.apache.hadoop.hbase.io.hfile.HFileBlockIndex$BlockIndexReader.seekToDataBlock(HFileBlockIndex.java:209)
at org.apache.hadoop.hbase.io.hfile.HFileReaderV2$ScannerV2.seekTo(HFileReaderV2.java:521)
at org.apache.hadoop.hbase.io.hfile.HFileReaderV2$ScannerV2.seekTo(HFileReaderV2.java:536)
at org.apache.hadoop.hbase.regionserver.StoreFileScanner.seekAtOrAfter(StoreFileScanner.java:178)
at org.apache.hadoop.hbase.regionserver.StoreFileScanner.seek(StoreFileScanner.java:111)
at org.apache.hadoop.hbase.regionserver.StoreFileScanner.seekExactly(StoreFileScanner.java:219)
at org.apache.hadoop.hbase.regionserver.StoreScanner.<init>(StoreScanner.java:80)
at org.apache.hadoop.hbase.regionserver.Store.getScanner(Store.java:1689)
at org.apache.hadoop.hbase.regionserver.HRegion$RegionScannerImpl.<init>(HRegion.java:2857)

Since the HFile name size is known and the offset is a long, it should be possible to allocate exactly what we need. Maybe use byte[] as the key and drop the separator too.