Remove usage of itertools (#15)

Itertools crate was used for ergonomic join on the iterator of strings.
Changed this to use join on the vec of strings from the std lib, and
droped a dependecy of the crate. Std lib join should even be more
performant, since it prealocates the string length correctly, while
itertools is doing the best effort it can.

Author: mvucenovic

Diff for: Cargo.lock

@@ -18,7 +18,6 @@ coveralls = { repository = "Empty2k12/influxdb-rust", branch = "master", service
 reqwest = "0.9.17"
 futures = "0.1.27"
 tokio = "0.1.20"
-itertools = "0.8"
 failure = "0.1.5"
 serde = { version = "1.0.92", optional = true }
 serde_json = { version = "1.0", optional = true }

Diff for: Cargo.toml

@@ -4,7 +4,6 @@
 
 use crate::error::InfluxDbError;
 use crate::query::{InfluxDbQuery, QueryType, Timestamp, ValidQuery};
-use itertools::Itertools;
 
 // todo: batch write queries
 
@@ -142,6 +141,7 @@ impl InfluxDbQuery for InfluxDbWriteQuery {
             .tags
             .iter()
             .map(|(tag, value)| format!("{tag}={value}", tag = tag, value = value))
+            .collect::<Vec<String>>()
             .join(",");
         if !tags.is_empty() {
             tags.insert_str(0, ",");
@@ -150,6 +150,7 @@ impl InfluxDbQuery for InfluxDbWriteQuery {
             .fields
             .iter()
             .map(|(field, value)| format!("{field}={value}", field = field, value = value))
+            .collect::<Vec<String>>()
             .join(",");
 
         Ok(ValidQuery(format!(