libs/db: conditional compilation (#3628)

* libs/db: conditional compilation

For cleveldb: go build -tags cleveldb
For boltdb: go build -tags boltdb

Fixes #3611

* document db_backend param better

* remove deprecated LevelDBBackend

* update changelog

* add missing lines

* add new line

* fix TestRemoteDB

* add a line about boltdb tag

* Revert "remove deprecated LevelDBBackend"

This reverts commit 1aa85453f7.

* make PR non breaking

* change DEPRECATED label format

https://stackoverflow.com/a/36360323/820520

docs/introduction/install.md

@@ -79,9 +79,7 @@ make install
 
 Install [LevelDB](https://github.com/google/leveldb) (minimum version is 1.7).
 
-### Ubuntu
-
-Install LevelDB with snappy (optionally):
+Install LevelDB with snappy (optionally). Below are commands for Ubuntu:
 
 ```
 sudo apt-get update
@@ -100,23 +98,23 @@ wget https://github.com/google/leveldb/archive/v1.20.tar.gz && \
   rm -f v1.20.tar.gz
 ```
 
-Set database backend to cleveldb:
+Set a database backend to `cleveldb`:
 
 ```
 # config/config.toml
 db_backend = "cleveldb"
 ```
 
-To install Tendermint, run
+To install Tendermint, run:
 
 ```
 CGO_LDFLAGS="-lsnappy" make install_c
 ```
 
-or run
+or run:
 
 ```
 CGO_LDFLAGS="-lsnappy" make build_c
 ```
 
-to put the binary in `./build`.
+which puts the binary in `./build`.