Add R API for `read.jdbc`, `write.jdbc`.
Tested this quite a bit manually with different combinations of parameters. It's not clear if we could have automated tests in R for this - Scala `JDBCSuite` depends on Java H2 in-memory database.
Refactored some code into util so they could be tested.
Core's R SerDe code needs to be updated to allow access to java.util.Properties as `jobj` handle which is required by DataFrameReader/Writer's `jdbc` method. It would be possible, though more code to add a `sql/r/SQLUtils` helper function.
Tested:
```
# with postgresql
../bin/sparkR --driver-class-path /usr/share/java/postgresql-9.4.1207.jre7.jar
# read.jdbc
df <- read.jdbc(sqlContext, "jdbc:postgresql://localhost/db", "films2", user = "user", password = "12345")
df <- read.jdbc(sqlContext, "jdbc:postgresql://localhost/db", "films2", user = "user", password = 12345)
# partitionColumn and numPartitions test
df <- read.jdbc(sqlContext, "jdbc:postgresql://localhost/db", "films2", partitionColumn = "did", lowerBound = 0, upperBound = 200, numPartitions = 4, user = "user", password = 12345)
a <- SparkR:::toRDD(df)
SparkR:::getNumPartitions(a)
[1] 4
SparkR:::collectPartition(a, 2L)
# defaultParallelism test
df <- read.jdbc(sqlContext, "jdbc:postgresql://localhost/db", "films2", partitionColumn = "did", lowerBound = 0, upperBound = 200, user = "user", password = 12345)
SparkR:::getNumPartitions(a)
[1] 2
# predicates test
df <- read.jdbc(sqlContext, "jdbc:postgresql://localhost/db", "films2", predicates = list("did<=105"), user = "user", password = 12345)
count(df) == 1
# write.jdbc, default save mode "error"
irisDf <- as.DataFrame(sqlContext, iris)
write.jdbc(irisDf, "jdbc:postgresql://localhost/db", "films2", user = "user", password = "12345")
"error, already exists"
write.jdbc(irisDf, "jdbc:postgresql://localhost/db", "iris", user = "user", password = "12345")
```
Author: felixcheung <felixcheung_m@hotmail.com>
Closes#10480 from felixcheung/rreadjdbc.
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