LIVY-245. Add support shared variables across Jobs

[jerryshao]

Currently we cannot share variables across different Jobs in Livy, so here propose to add a cache layer in RSC to store shared objects. This cache followed LRU, the least not used will be removed when exceeding limits.

This work is based on @alex-the-man 's work #248

[codecov-io]

Codecov Report

Merging #333 into master will increase coverage by 0.18%.
The diff coverage is 88.46%.

@@             Coverage Diff              @@
##             master     #333      +/-   ##
============================================
+ Coverage     70.46%   70.65%   +0.18%     
- Complexity      726      731       +5     
============================================
  Files            96       96              
  Lines          5123     5145      +22     
  Branches        774      776       +2     
============================================
+ Hits           3610     3635      +25     
+ Misses          996      994       -2     
+ Partials        517      516       -1

Impacted Files	Coverage Δ	Complexity Δ
...a/com/cloudera/livy/scalaapi/ScalaJobContext.scala	23.07% <0%> (-6.93%)	0 <0> (ø)
...rc/main/scala/com/cloudera/livy/repl/Session.scala	74.34% <100%> (ø)	1 <0> (ø)	⬇️
...c/src/main/java/com/cloudera/livy/rsc/RSCConf.java	87.75% <100%> (+0.12%)	7 <0> (ø)	⬇️
...a/com/cloudera/livy/rsc/driver/JobContextImpl.java	69.01% <100%> (+10.52%)	19 <4> (+4)	⬆️
...c/src/main/java/com/cloudera/livy/rsc/rpc/Rpc.java	78.61% <0%> (+0.62%)	12% <0%> (ø)	⬇️
...n/java/com/cloudera/livy/rsc/driver/RSCDriver.java	77.15% <0%> (+0.86%)	39% <0%> (ø)	⬇️
rsc/src/main/java/com/cloudera/livy/rsc/Utils.java	85.71% <0%> (+2.38%)	16% <0%> (ø)	⬇️
...va/com/cloudera/livy/rsc/rpc/KryoMessageCodec.java	98.11% <0%> (+3.77%)	19% <0%> (+1%)	⬆️

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