ISSUE-2640: BP-43 Integrate spotbugs plugin with gradle

[pkumar-singh]

Motivation

Migrate bookkeeper to gradle spotbug plugin.
spotbugs is gradle as well as maven plugin which checks for obvious bugs in source code.
How to run
./gradlew spotbugsMain

Changes

• Integrate gradle build with spotbugs plugin
• Second commit is for fixing issues reported by spotbugs plugin

Test

Just to validate it's working.
I introduced following bug.

public void test() {
+
+        String[] field = {"a", "b", "c", "s", "e"};
+
+        //concatenates strings using + in a loop
+        String ip = "127.0.0.1";
+        String s = "";
+        for (int i = 0; i < field.length; ++i) {
+            s = s + field[i];
+        }
+
+        System.out.println(ip);
+
+    }

spotbugs plugin reported following violation.

<BugInstance type="SBSC_USE_STRINGBUFFER_CONCATENATION" priority="2" rank="18" abbrev="SBSC" category="PERFORMANCE" instanceHash="2aa8e4ebcc4cd86190fe3e7bd8a4913c" instanceOccurrenceNum="0" instanceOccurrenceMax="0">
    <ShortMessage>Method concatenates strings using + in a loop</ShortMessage>
    <LongMessage>org.apache.bookkeeper.meta.LongZkLedgerIdGenerator.test() concatenates strings using + in a loop</LongMessage>
    <Class classname="org.apache.bookkeeper.meta.LongZkLedgerIdGenerator" primary="true">
      <SourceLine classname="org.apache.bookkeeper.meta.LongZkLedgerIdGenerator" start="58" end="350" sourcefile="LongZkLedgerIdGenerator.java" sourcepath="org/apache/bookkeeper/meta/LongZkLedgerIdGenerator.java" relSourcepath="java/org/apache/bookkeeper/meta/LongZkLedgerIdGenerator.java">
        <Message>At LongZkLedgerIdGenerator.java:[lines 58-350]</Message>
      </SourceLine>
      <Message>In class org.apache.bookkeeper.meta.LongZkLedgerIdGenerator</Message>
    </Class>

Master Issue: #2640

[eolivelli]

overall +1
I left one comment about the version of spotbugs gradle vs Maven

[hsaputra]

LGTM +1

[eolivelli]

+1

[merlimat]

@pkumar-singh You might need to rebase on master to get fixes for CI

[hsaputra]

The failed tests seems coming from timeouts and repeating fails in other PRs.
Since the change is related gradle changes, will merge this one.