JavaIdentifierNormalizer ThreadSafe/Memory leak fix

JavaIdentifierNormalizer ThreadSafe/Memory leak fix

1. make it thread-safe
2. memory leak fix
3. avoid deep indentations by obeying code conventions

Result "org.opendaylight.coretutorials.MyBenchmark.testBindingV2":
  165.567 ±(99.9%) 1.548 ops/s [Average]
  (min, avg, max) = (146.161, 165.567, 180.523), stdev = 6.553
  CI (99.9%): [164.019, 167.115] (assumes normal distribution)

Benchmark                   Mode  Cnt    Score   Error  Units
MyBenchmark.testBindingV2  thrpt  200  165.567 ± 1.548  ops/s

We're still around 2.8x slower than binding v1 generator,
so there will be follow-up activity to analyse/profile
architecture vs. implementation weakness and fixes
should/might be necessary to improve binding v2 generator.

Change-Id: Id1ca6e5f530c0340c3234526d6babf1f9e3c62b0
Signed-off-by: Martin Ciglan <martin.ciglan@pantheon.tech>