Improve the PCRE cache

[tspivey]

This fixes the following problems with the cache:

1. It would crash the server if it was used from multiple threads.
2. The case matters option to pcre_match wasn't being taken into account, and the cache would match the wrong pattern. Example:

   ;pcre_match("test", "e", 0)
   => {["0" -> ["match" -> "e", "position" -> {2, 2}]]}
   ;pcre_match("TEST", "e", 1)
   => {["0" -> ["match" -> "E", "position" -> {2, 2}]]}

With this PR applied:

;pcre_match("test", "e", 0)
=> {["0" -> ["match" -> "e", "position" -> {2, 2}]]}
;pcre_match("TEST", "e", 1)
=> {}

[ctoth]

This closes #79