Update jackson-databind version

**Feature:**

Update jackson-databind version in zipkin-server-*-exec.jar to fix CVEs



**Rational**

Hello, I noticed that current zipkin-server-*-exec.jar includes com.fasterxml.jackson.core:jackson-databind:jar:2.7.9.3 that has numerous CVEs including the one that was previously fixed in https://github.com/openzipkin/zipkin/pull/2771 . It looks like it was caused by cassandra-driver-core update to 3.8.0 (https://github.com/openzipkin/zipkin/commit/325e39703152a0c587df7ca229ae88852a35b2e7#diff-600376dffeb79835ede4a0b285078036 ) since it now comes with jackson-databind: 



```

[INFO] io.zipkin.zipkin2:zipkin-storage-cassandra:jar:2.19.3-SNAPSHOT

[INFO] +- com.google.auto.value:auto-value-annotations:jar:1.7:compile

[INFO] +- com.google.auto.value:auto-value:jar:1.7:provided

[INFO] +- com.datastax.cassandra:cassandra-driver-core:jar:3.8.0:compile

[INFO] |  +- io.netty:netty-handler:jar:4.0.56.Final:compile

[INFO] |  |  +- io.netty:netty-buffer:jar:4.0.56.Final:compile

[INFO] |  |  |  \- io.netty:netty-common:jar:4.0.56.Final:compile

[INFO] |  |  +- io.netty:netty-transport:jar:4.0.56.Final:compile

[INFO] |  |  \- io.netty:netty-codec:jar:4.0.56.Final:compile

[INFO] |  +- com.google.guava:guava:jar:19.0:compile

[INFO] |  +- io.dropwizard.metrics:metrics-core:jar:3.2.2:compile

[INFO] |  +- org.slf4j:slf4j-api:jar:1.7.28:compile

[INFO] |  +- com.github.jnr:jnr-ffi:jar:2.1.7:compile

[INFO] |  |  +- com.github.jnr:jffi:jar:1.2.16:compile

[INFO] |  |  +- com.github.jnr:jffi:jar:native:1.2.16:runtime

[INFO] |  |  +- org.ow2.asm:asm:jar:5.0.3:compile

[INFO] |  |  +- org.ow2.asm:asm-commons:jar:5.0.3:compile

[INFO] |  |  +- org.ow2.asm:asm-analysis:jar:5.0.3:compile

[INFO] |  |  +- org.ow2.asm:asm-tree:jar:5.0.3:compile

[INFO] |  |  +- org.ow2.asm:asm-util:jar:5.0.3:compile

[INFO] |  |  \- com.github.jnr:jnr-x86asm:jar:1.0.2:compile

[INFO] |  +- com.github.jnr:jnr-posix:jar:3.0.44:compile

[INFO] |  |  \- com.github.jnr:jnr-constants:jar:0.9.9:compile

[INFO] |  \- com.fasterxml.jackson.core:jackson-databind:jar:2.7.9.3:compile

[INFO] |     +- com.fasterxml.jackson.core:jackson-annotations:jar:2.7.0:compile

[INFO] |     \- com.fasterxml.jackson.core:jackson-core:jar:2.7.9:compile

[INFO] +- com.datastax.cassandra:cassandra-driver-mapping:jar:3.8.0:compile

```



Updating cassandra-driver-core should also change jackson-databind version( https://github.com/openzipkin/zipkin/issues/2405 ), but for now maybe it is possible to revert cassandra-driver-core version to 3.7.2 ? Or maybe specify jackson-databind version directly?

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