Add TLS support to the benchmark script

[kpumuk]

TLS support added to the benchmark script, can be enabled with THRIFT_TLS=true environment variable. Used to produce benchmark results for #3243

Uses keys from test/keys, shared with cross tests.

Usage:

$ THRIFT_TLS=true ruby benchmark/benchmark.rb
Starting server...
Spawning benchmark processes...
Collecting output...
Translating output...
Analyzing output...

Server class:        Thrift::ThreadPoolServer
Server interpreter:  ruby
Client interpreter:  ruby
Socket class:        Thrift::SSLSocket
Number of processes: 4
Clients per process: 100
Calls per client:    50
Using fastthread:    no

Connection failures:                0
Connection errors:                  0
Average time per call:              0.0005 seconds
Average time per client (50 calls): 0.0361 seconds
Total time for all calls:           9.0603 seconds
Real time for benchmarking:         4.2191 seconds
Shortest call time:                 0.0001 seconds
Longest call time:                  0.0136 seconds
Shortest client time (50 calls):    0.0260 seconds
Longest client time (50 calls):     0.0573 seconds

• Did you create an Apache Jira ticket? (Request account here, not required for trivial changes)
• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• Did you squash your changes to a single commit? (not required, but preferred)
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.