Simplify and clarify cmake script and build instruction

[gavinchou]

Replace include(FindProtobuf) with "manual detection" of protobuffer because:

1. We can make the names of dependencies consistent in a form of XXX_INCLUDE_PATH and XXX_LIB. Before replacing, we may have to provide Protobuf_INCLUDE_DIR and Protobuf_LIBRARY too, which is inconsistent with other libs.
2. If the protoc's version, the protoc is detected by include(FindProtobuf), is different (there are several versions installed) from the libprotobuf provided, it may lead to version inconsistency.

[PFZheng]

Replace include(FindProtobuf) with "manual detection" of protobuffer because:

1. We can make the names of dependencies consistent in a form of XXX_INCLUDE_PATH and XXX_LIB. Before replacing, we may have to provide Protobuf_INCLUDE_DIR and Protobuf_LIBRARY too, which is inconsistent with other libs.
2. If the protoc's version, the protoc is detected by include(FindProtobuf), is different (there are several versions installed) from the libprotobuf provided, it may lead to version inconsistency.

Please check travis-ci

[Edward-xk]

-- Found Threads: TRUE
CMake Error at /usr/local/cmake-3.12.4/share/cmake-3.12/Modules/FindProtobuf.cmake:425 (file): file STRINGS file "/home/travis/build/brpc/braft/PROTOBUF_INCLUDE_PATH/google/protobuf/stubs/common.h" cannot be read. Call Stack (most recent call first): CMakeLists.txt:33 (include)

[jovany-wang]

A nice PR!