Avoid linking binary with unused libraries

[d-uspenskiy]

Jira Link: DB-1441
there are lots of dependencies for each binary in CMakeFile. Here is a list of dependencies for yb-tserver

../../../lib/libyb-cql.so ../../../lib/libyb-redis.so ../../../lib/libmaster.so ../../../../../thirdparty/installed/common/lib/libunwind.so ../../../lib/libql_api.so ../../../lib/libql_sem.so ../../../lib/libql_exec.so ../../../lib/libql_parser.so ../../../lib/libql_ptree.so ../../../lib/libql_util.so ../../../lib/libcql_service_proto.so ../../../../../thirdparty/installed/common/lib/liblz4.a ../../../lib/libredis_service_proto.so ../../../lib/libtserver.so ../../../lib/libtablet.so ../../../lib/libconsensus.so ../../../lib/liblog.so ../../../lib/liblog_proto.so ../../../lib/libyb_docdb.so ../../../lib/libyb_pggate_util.so ../../../lib/libyb_client.so ../../../lib/libtserver_service_proto.so ../../../lib/libremote_bootstrap_proto.so ../../../lib/libconsensus_proto.so ../../../lib/libbackup_proto.so ../../../lib/libtserver_admin_proto.so ../../../lib/libmaster_rpc.so ../../../lib/libserver_process.so ../../../../../thirdparty/installed/uninstrumented/lib/libprofiler.so ../../../lib/libserver_base_proto.so ../../../../../thirdparty/installed/common/lib/libsqueasel.a ../../../lib/libtserver_util.so ../../../lib/libyb_docdb_encoding.so ../../../lib/libserver_common.so ../../../lib/libyb_fs.so ../../../lib/libyb_rocksutil.so ../../../lib/librocksdb.so ../../../../../thirdparty/installed/uninstrumented/lib/libsnappy.so -lbz2 -lz ../../../lib/libmaster_util.so ../../../lib/libmaster_backup_proto.so ../../../lib/libmaster_proto.so ../../../lib/libyb_pgwrapper.so ../../../lib/libyb_pggate_flags.so ../../../lib/libtserver_proto.so ../../../lib/libdocdb_proto.so ../../../lib/libyrpc.so ../../../lib/librpc_introspection_proto.so ../../../../../thirdparty/installed/common/lib/libev.so /home/duspensky/.linuxbrew-yb-build/linuxbrew-20181203T161736/lib/libssl.so ../../../lib/libtablet_proto.so ../../../lib/libyb_common.so ../../../lib/libredis_protocol_proto.so ../../../lib/libpgsql_protocol_proto.so ../../../lib/libql_protocol_proto.so ../../../lib/libyb_bfql.so ../../../lib/libyb_bfpg.so ../../../lib/libyb_util.so ../../../../../thirdparty/installed/uninstrumented/lib/libtcmalloc.so ../../../../../thirdparty/installed/uninstrumented/lib/libboost_system.so ../../../../../thirdparty/installed/uninstrumented/lib/libboost_thread.so ../../../lib/libgutil.so ../../../../../thirdparty/installed/uninstrumented/lib/libglog.so ../../../../../thirdparty/installed/uninstrumented/lib/libgflags.so /home/duspensky/.linuxbrew-yb-build/linuxbrew-20181203T161736/lib/libcrypto.so ../../../../../thirdparty/installed/uninstrumented/lib/libcds.so /home/duspensky/.linuxbrew-yb-build/linuxbrew-20181203T161736/lib/libicui18n.so /home/duspensky/.linuxbrew-yb-build/linuxbrew-20181203T161736/lib/libicuuc.so ../../../../../thirdparty/installed/uninstrumented/lib/libcrcutil.so ../../../../../thirdparty/installed/uninstrumented/lib/libcrypt_blowfish.a ../../../lib/libhistogram_proto.so ../../../lib/libpb_util_proto.so ../../../lib/libencryption_proto.so ../../../../../thirdparty/installed/common/lib/libz.so /home/duspensky/.linuxbrew-yb-build/linuxbrew-20181203T161736/lib/libcrypto.so /home/duspensky/.linuxbrew-yb-build/linuxbrew-20181203T161736/lib/librt.so ../../../../../thirdparty/installed/uninstrumented/lib/libbacktrace.so ../../../../../thirdparty/installed/common/lib/libcurl.so -ldl -luuid ../../../lib/libopid_proto.so ../../../lib/libwire_protocol_proto.so ../../../lib/libconsensus_metadata_proto.so ../../../lib/libyb_common_proto.so ../../../lib/libfs_proto.so ../../../lib/librpc_header_proto.so ../../../lib/libversion_info_proto.so ../../../../../thirdparty/installed/uninstrumented/lib/libprotobuf.so

without --as-needed flag all of these shared library will be linked to binary even in case they are not used.
This can be checked by

[duspensky@centos bin]$ ldd -u -r yb-tserver 
Unused direct dependencies:
    /home/duspensky/.linuxbrew-yb-build/linuxbrew-20181203T161736-xxxxxxxxxxxxxxxxxxxxxxx/lib/libatomic.so.1
    /home/duspensky/code/yugabyte/build/debug-gcc-dynamic-enterprise/bin/./../../../thirdparty/installed/common/lib/libunwind.so.8
    /home/duspensky/code/yugabyte/build/debug-gcc-dynamic-enterprise/bin/./../lib/libql_api.so
    /home/duspensky/code/yugabyte/build/debug-gcc-dynamic-enterprise/bin/./../lib/libql_sem.so
    /home/duspensky/code/yugabyte/build/debug-gcc-dynamic-enterprise/bin/./../lib/libql_exec.so
    /home/duspensky/code/yugabyte/build/debug-gcc-dynamic-enterprise/bin/./../lib/libql_parser.so
    /home/duspensky/code/yugabyte/build/debug-gcc-dynamic-enterprise/bin/./../lib/libql_ptree.so
    /home/duspensky/code/yugabyte/build/debug-gcc-dynamic-enterprise/bin/./../lib/libql_util.so
    /home/duspensky/code/yugabyte/build/debug-gcc-dynamic-enterprise/bin/./../lib/libcql_service_proto.so
...
    /home/duspensky/code/yugabyte/build/debug-gcc-dynamic-enterprise/bin/./../lib/libyb_pggate_flags.so
...

but in case '--as-needed' is specified resulting binary will not contain unused dependencies (this will speedup loading process)