Update CHANGELOG and version in prep for 1.20.0 release (uber#805)

## [1.20.0] - 2020-09-23
### Added
 * Support per-connection base context propagation in inbound/outbound connections and relays (uber#801)

### Changed
 * Internal API changes related to relaying.

CHANGELOG.md

@@ -1,6 +1,13 @@
 Changelog
 =========
 
+## [1.20.0] - 2020-09-23
+### Added
+ * Support per-connection base context propagation for inbound/outbound connections (#801)
+
+### Changed
+ * Internal API changes related to relaying.
+
 ## [1.19.1] - 2020-08-03
 ### Fixed
  * Move OS-specific logic into OS-specific files to avoid compile issues on
@@ -295,6 +302,7 @@ Changelog
 * Thrift support, including includes.
 
 [//]: # (Version Links)
+[1.20.0]: https://github.com/uber/tchannel-go/compare/v1.19.1...v1.20.0
 [1.18.0]: https://github.com/uber/tchannel-go/compare/v1.17.0...v1.18.0
 [1.17.0]: https://github.com/uber/tchannel-go/compare/v1.16.0...v1.17.0
 [1.16.0]: https://github.com/uber/tchannel-go/compare/v1.15.0...v1.16.0

version.go

@@ -23,4 +23,4 @@ package tchannel
 // VersionInfo identifies the version of the TChannel library.
 // Due to lack of proper package management, this version string will
 // be maintained manually.
-const VersionInfo = "1.19.1-dev"
+const VersionInfo = "1.20.0"