Cache proto objects

[kolodny]

Creating proto objects are incredibly expensive due to the usage of loadProtoFile_ which eventually calls readFileSync.

This PR keeps track of a cache to minimize those calls. Try the following before and after the change:

const BigTable = require('@google-cloud/bigtable');

for (let i = 0; i < 20; i++) {
  console.time(i);
  const bigtable = BigTable();
  console.timeEnd(i);
}

Before:

0: 61.598ms
1: 42.123ms
2: 35.100ms
3: 47.477ms
4: 34.963ms
5: 45.671ms
6: 23.578ms
7: 36.888ms
8: 39.864ms
9: 27.422ms
10: 24.460ms
11: 38.407ms
12: 33.231ms
13: 33.515ms
14: 31.255ms
15: 38.172ms
16: 23.016ms
17: 33.301ms
18: 30.991ms
19: 43.421ms

After:

0: 48.167ms
1: 0.276ms
2: 0.144ms
3: 0.141ms
4: 0.089ms
5: 0.133ms
6: 0.089ms
7: 0.077ms
8: 0.103ms
9: 0.115ms
10: 0.129ms
11: 0.086ms
12: 0.077ms
13: 0.119ms
14: 0.093ms
15: 0.077ms
16: 0.125ms
17: 0.127ms
18: 0.117ms
19: 0.206ms

[coveralls]

Coverage remained the same at 100.0% when pulling a6b7e2b on kolodny:cache-proto-objects into b8a00b2 on GoogleCloudPlatform:master.

[stephenplusplus]

LGTM! Thanks, @kolodny!