fix: AmazonS3KeyValueAccess.isFile

[bogovicj]

• create prefixExists which should be used by isDirectory
• changed implementation of keyExists, used by isFile
• no longer returns true erroneously when an existing key has the queried key as a prefix (e.g. used to return true if 9.1 does not exist but 9.10 does)

@cmhulbert @axtimwalde

[bogovicj]

This also fixes exceptions we would see like this the one below cased by
AmazonS3KeyValueAccess.exists erroneously returning true.

Caused by: com.amazonaws.services.s3.model.AmazonS3Exception: The specified key does not exist. (Service: Amazon S3; Status Code: 404; Error Code: NoSuchKey; Request ID: 5ZZKCMTMQA6G3YBC; S3 Extended Request ID: 3pLOql4JbkaXuKB8NyM0mJXRfpBvd+rdt/YWbuGOpj7zlL88jZ3aVq2PePxwDO87mhsYfBH2mRM=; Proxy: null), S3 Extended Request ID: 3pLOql4JbkaXuKB8NyM0mJXRfpBvd+rdt/YWbuGOpj7zlL88jZ3aVq2PePxwDO87mhsYfBH2mRM=
	at com.amazonaws.http.AmazonHttpClient$RequestExecutor.handleErrorResponse(AmazonHttpClient.java:1879)
	at com.amazonaws.http.AmazonHttpClient$RequestExecutor.handleServiceErrorResponse(AmazonHttpClient.java:1418)
	at com.amazonaws.http.AmazonHttpClient$RequestExecutor.executeOneRequest(AmazonHttpClient.java:1387)
	at com.amazonaws.http.AmazonHttpClient$RequestExecutor.executeHelper(AmazonHttpClient.java:1157)
	at com.amazonaws.http.AmazonHttpClient$RequestExecutor.doExecute(AmazonHttpClient.java:814)
	at com.amazonaws.http.AmazonHttpClient$RequestExecutor.executeWithTimer(AmazonHttpClient.java:781)
	at com.amazonaws.http.AmazonHttpClient$RequestExecutor.execute(AmazonHttpClient.java:755)
	at com.amazonaws.http.AmazonHttpClient$RequestExecutor.access$500(AmazonHttpClient.java:715)
	at com.amazonaws.http.AmazonHttpClient$RequestExecutionBuilderImpl.execute(AmazonHttpClient.java:697)
	at com.amazonaws.http.AmazonHttpClient.execute(AmazonHttpClient.java:561)
	at com.amazonaws.http.AmazonHttpClient.execute(AmazonHttpClient.java:541)
	at com.amazonaws.services.s3.AmazonS3Client.invoke(AmazonS3Client.java:5470)
	at com.amazonaws.services.s3.AmazonS3Client.invoke(AmazonS3Client.java:5417)
	at com.amazonaws.services.s3.AmazonS3Client.getObject(AmazonS3Client.java:1525)
	at com.amazonaws.services.s3.AmazonS3Client.getObject(AmazonS3Client.java:1379)
	at org.janelia.saalfeldlab.n5.s3.AmazonS3KeyValueAccess$S3ObjectChannel.newInputStream(AmazonS3KeyValueAccess.java:559)
	at org.janelia.saalfeldlab.n5.zarr.ZarrKeyValueReader.readBlock(ZarrKeyValueReader.java:638)
	at org.janelia.saalfeldlab.n5.imglib2.N5CacheLoader.get(N5CacheLoader.java:120)
	at org.janelia.saalfeldlab.n5.imglib2.N5CacheLoader.get(N5CacheLoader.java:89)
	at net.imglib2.cache.ref.SoftRefLoaderCache.get(SoftRefLoaderCache.java:135)