Performance issue due to inefficient ObjectMapper initialization

Found few performance bottlenecks on benchmarking PostgreSQL CDC events. This Issue will focus on improving the ObjectMapper initialization (others will be filed as separate Issues).

`io.debezium.connector.postgresql.SourceInfo.sequence()` has a major performance bottleneck due to Initialization of `ObjectMapper` on every call (i.e., for every cdc row event). Pull Request will be raised with optimizing the `new ObjectMapper()` to `private static final` field.

JVisualVM Sampler metrics attached for 1M row CDC events on a table with 4 columns (NOOP DebeziumEngine.notifying)

Overall Performance improvement can range from approx `5% - 15%` depending upon row width (no of columns)

JMH Benchmarks

Benchmark                                        Mode  Cnt        Score        Error  Units
PostgresConnectorConfigPerf.benchmark_1_before  thrpt    3   550040.596 ±  16847.245  ops/s
PostgresConnectorConfigPerf.benchmark_2_after   thrpt    3  4347346.228 ± 108381.634  ops/s